Updated 3 hours ago · Specifications verified October 12, 2026

Fill Two Bays: DS225+ and DS725+

Two bays, 24TB per bay on Synology's list, 24TB usable in SHR

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Quick answer

The DS225+ and DS725+ each have two SATA bays, and Synology's compatibility list for both goes up to 24TB per drive, so the internal bays hold 48TB raw. The DS225+ lists no expansion unit. On the DS725+, one DX525 expansion unit takes it to 7 bays. Start from SHR, one drive of protection: two 24TB drives then give 24TB of space. The list contains 22 hard drives, all Synology's own; from DSM 7.3 both models also build storage pools on third-party NAS drives, which are the cheaper table below.

As of October 12, 2026

Best price per TB

$62.50

In-stock listings in the tables on this page

Prices updated

3 hours ago

What two bays hold on the DS225+ and DS725+

Drive size	Raw (2 drives)	Usable, one drive of protection
8TB	16TB	8TB
12TB	24TB	12TB
16TB	32TB	16TB
20TB	40TB	20TB
24TB	48TB	24TB

Where the DS225+ and DS725+ differ

	DS225+	DS725+
Most bays with expansion	Not expandable	7 (DX525 × 1)
RAID types that need an expansion unit	None	RAID 5, RAID 6, RAID 10
Largest single volume	108TB	200TB with 32GB of memory, otherwise 108TB
M.2 slots	None	2 x M.2 2280
Power draw in use	16.98W	20.41W
Noise	19.6 dB(A)	17.9 dB(A)
Processor	Intel Celeron J4125, 4 cores	AMD Ryzen R1600, 2 cores / 4 threads

The layout to start from with two bays

With two bays there is one sensible protected layout: both drives hold the same data, so you keep one drive's capacity and survive one failure.

A full set of 24TB drives in SHR gives 24TB, which is inside the 108TB single-volume limit on both models, so the limit never constrains the internal bays.

RAID 5, RAID 6, RAID 10 appear on the DS725+'s RAID list only with an expansion unit attached, because they need more drives than its two internal bays.

Hard drives on Synology's list for the DS225+ and DS725+

Series	Capacities listed
HAT3300 (Plus)	2, 4, 6, 8, 12TB
HAT3310 (Plus)	8, 12, 16TB
HAT3320 (Plus)	8, 20TB
HAT5300 (Enterprise)	4, 8, 12, 16TB
HAT5310 (Enterprise)	8, 16, 18, 20TB
HAT5320 (Enterprise)	4, 8, 20, 24TB

Where the DS225+ and DS725+ figures come from

Read from Synology's DS225+ and DS725+ product pages and compatibility lists on October 12, 2026. Capacities are bays times drive size in decimal terabytes.

Checked on October 12, 2026. Sources: Synology DS225+ technical specifications; Synology DS725+ technical specifications; Synology compatibility list for the DS225+; Synology compatibility list for the DS725+; Synology Knowledge Center: Drive compatibility policies for systems from 2025 (DSM 7.3 and above).

Background this page does not repeat

Only DS225+ and DS725+ specifics are on this page.

NAS drive basics: CMR, NAS-rated drives and SHR · Synology DSM 7.3 third-party drive policy

The DS225+ on Amazon.com today

New listings of the DS225+ itself, including bundles sold with drives fitted, which are labelled.

Price per

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7 products

Product	Capacity	Price	$ / TB	Type	Prime	Buy
Synology DS225+ Private Cloud Media Server - Stream, Back Up Photos & Share Files, Intel CPU for Hardware Transcoding (2-Bay Diskless NAS)Sold by Amazon	Diskless	$364.99	-	NAS device · network	Prime	Buy on Amazon
Synology DS225+ 8TB Bundle Best Price HDDs 2X 4TB 2-Bay NAS Plus Series - Intel Celeron J4125 Quad-Core, 2GB Non-ECC DDR4 SODIMM 1 x 2.5 GbE and 1 x 1 GbE RJ-45 LAN Port	8 TB included (2 x 4 TB)	$949.00	-	NAS + drives bundle	-	Buy on Amazon
Synology DS225+ 2-Bay Diskstation NAS (Intel Celeron J4125 Quad-Core, 2 GB Non-ECC DDR4 SODIMM 1 x 2,5 GbE und 1 x 1 GbE RJ-45 LAN-Port) 8TB Bundle with 2X 4TB Seagate Ironwolf	8 TB included (2 x 4 TB)	$981.42	-	NAS + drives bundle	-	Buy on Amazon
Synology DS225+ 2-Bay Diskstation NAS (Intel Celeron J4125 Quad-Core, 2 GB Non-ECC DDR4 SODIMM 1 x 2,5 GbE und 1 x 1 GbE RJ-45 LAN-Port) 16TB Bundle with 2X 8TB Seagate Ironwolf	16 TB included (2 x 8 TB)	$1,412.93	-	NAS + drives bundle	-	Buy on Amazon
Synology DS225+ 20TB Bundle Best Price HDDs 2X 10TB 2-Bay NAS Plus Series - Intel Celeron J4125 Quad-Core, 2GB Non-ECC DDR4 SODIMM 1 x 2.5 GbE and 1 x 1 GbE RJ-45 LAN Port	20 TB included (2 x 10 TB)	$1,499.00	-	NAS + drives bundle	-	Buy on Amazon
Synology DS225+ 2-Bay Diskstation NAS (Intel Celeron J4125 Quad-Core, 2 GB Non-ECC DDR4 SODIMM 1 x 2,5 GbE und 1 x 1 GbE RJ-45 LAN-Port) 24TB Bundle with 2X 12TB Seagate Ironwolf	24 TB included (2 x 12 TB)	$1,599.00	-	NAS + drives bundle	-	Buy on Amazon
Synology DS225+ 24TB (Synology HAT3310-12T) 2-Bay Plus Series - Intel Celeron J4125 Quad-Core, 2GB Non-ECC DDR4 SODIMM 1x2.5GbE and 1x1GbE RJ-45 LAN Port	24 TB included	$1,799.00	-	NAS + drives bundle	-	Buy on Amazon

The DS725+ on Amazon.com today

New listings of the DS725+ itself, including bundles sold with drives fitted, which are labelled.

Price per

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2 products

Product	Capacity	Price	$ / TB	Type	Prime	Buy
Synology DS725+ Expandable NAS Storage - Private Cloud for Home and Small Business (2-Bay Diskless NAS)Sold by Amazon	Diskless	$527.99	-	NAS device · network	Prime	Buy on Amazon
Synology DS725+ 2-Bay Diskstation NAS (Ryzen R1600, 4GB Ram), 1x 2.5GbE, 1x 1GbE, 2X M.2 NVMe Steckplätze 8TB Bundle with 2X 4TB Seagate Ironwolf	8 TB included (2 x 4 TB)	$1,315.22	-	NAS + drives bundle	-	Buy on Amazon

Drives on Synology's DS225+ and DS725+ list, in stock

Synology HAT series drives, the only hard drives on Synology's compatibility list for the DS225+ and DS725+.

Price per

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1 products

Product	Capacity	Price	$ / TB	Type	Prime	Buy
Synology HAT5300-16T 16 TB Hard Drive - 3.5" Internal - SATA (SATA/600)	16 TB	$999.99	$62.50	NAS HDD · SATACMR	-	Buy on Amazon

Third-party NAS drives up to 24TB, by price per TB

Not on Synology's list. Supported for storage pools on the DS225+ and DS725+ from DSM 7.3.

Price per

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60 products

Product	Capacity	Price	$ / TB	Type	Prime	Buy
TOSHIBA N300 Internal Hard Drive 22 Tb 7200 RPM 512 Mb 3.5", W129161818 (Tb 7200 RPM 512 Mb 3.5 Serial Ata Iii)	22 TB	$799.99	$36.36	NAS HDD · SATACMR	-	Buy on Amazon
DC HC550 16TB 512MB SATA Ultra SE NP3	16 TB	$589.45	$36.84	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro 14 TB NAS RAID Internal Hard Drive - 7,200 RPM SATA 6 Gb/s 3.5-inch (ST14000NE0008)	14 TB	$521.00	$37.21	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf 14TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 256MB Cache for RAID Network Attached Storage – Frustration Free Packaging (ST14000VN0008)	14 TB	$525.00	$37.50	NAS HDD · SATACMR	-	Buy on Amazon
Toshiba - HDWG82AXZSTB - Toshiba N300 Pro HDWG82AXZSTB 20 TB Hard Drive - 3.5 Internal - SATA (SATA/600) - Conventional	20 TB	$766.99	$38.35	NAS HDD · SATACMR	-	Buy on Amazon
Toshiba MG Series Enterprise 12TB 3.5’’ SATA 6Gbit/s Internal HDD 7200RPM 550TB/year 24/7 Operation. MG07ACA12TE	12 TB	$469.99	$39.17	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro, 16 TB, Enterprise NAS Internal HDD –CMR 3.5 Inch, SATA 6 Gb/s, 7,200 RPM, 256 MB Cache for RAID Network Attached Storage (ST16000NT001)	16 TB	$629.99	$39.37	NAS HDD · SATACMR	-	Buy on Amazon
Seagate Enterprise 3.5" 18000 GB 7200 RPM 18 TB 3.5" 7200 RPM SATA ST18000NM00J	18 TB	$712.00	$39.56	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Enterprise Capacity \| ST10000NM0016 \| 10TB 7200 RPM SATA 6.0Gb/s 256MB Cache Hyperscale 512e 3.5" Hard Disk Drives	10 TB	$399.95	$39.99	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Exos X10 ST10000NM0126 10TB 7200 RPM SATA 6Gb/s 512e 256MB 3.5in Enterprise Hard Drive	10 TB	$399.99	$40.00	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro ST14000NE0008 14 TB Hard Drive - 3.5" Internal - SATA (SATA/600)	14 TB	$565.92	$40.42	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital WUH721414ALE6L4 14TB Ultrastar DC HC530 SATA HDD - 7200 RPM Class, SATA 6 Gb/s, 512MB Cache, 3.5"	14 TB	$581.00	$41.50	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro, 20 TB, Enterprise NAS Internal HDD –CMR 3.5 Inch, SATA 6 Gb/s, 7,200 RPM, 256 MB Cache for RAID Network Attached Storage (ST20000NT001)	20 TB	$829.99	$41.50	NAS HDD · SATACMR	-	Buy on Amazon
Seagate 20TB Exos Enterprise Hard Drive \| SATA (ST20000NM002H)	20 TB	$829.99	$41.50	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Toshiba - HDWG81GXZSTB - Toshiba N300 Pro HDWG81GXZSTB 16 TB Hard Drive - 3.5 Internal - SATA (SATA/600) - Conventional	16 TB	$685.50	$42.84	NAS HDD · SATACMR	-	Buy on Amazon
Seagate 14TB HDD Exos X14 7200 RPM 512e/4Kn SATA 6Gb/s 256MB Cache 3.5-Inch Enterprise Hard Drive (ST14000NM0018)	14 TB	$600.00	$42.86	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
HGST WD Ultrastar DC HC510 10TB 7200 RPM SATA 6Gb/s 3.5" Helium Platform Enterprise Hard Disk Drive - HUH721010ALE604 (0F27606)	10 TB	$429.00	$42.90	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Exos X18 Enterprise Class, 16TB, Enterprise Internal Hard Drive, SATA, 3,5" 6Gbit/s, 128MB Cache (ST16000NM000J)	16 TB	$686.91	$42.93	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 22TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD221KFGXSold by Amazon	22 TB	$958.88	$43.59	NAS HDD · SATACMR	Prime	Buy on Amazon
Toshiba 22TB Enterprise Internal Hard Drive – MG Series 3.5" SATA HDD for Server, Storage, 24/7 Operation, Hyperscale, Cloud (MG10AFA22TE)	22 TB	$959.95	$43.63	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 20TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD202KFGXSold by Amazon	20 TB	$880.96	$44.05	NAS HDD · SATACMR	Prime	Buy on Amazon
Western Digital 18TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD181KFGX	18 TB	$793.99	$44.11	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 24TB WD Gold Enterprise Class SATA Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, 512 MB Cache, 3.5" - WD242KRYZSold by Amazon	24 TB	$1,059.99	$44.17	Enterprise HDD · SATACMREnterprise	Prime	Buy on Amazon
Seagate IronWolf Pro 14TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 6Gb/s 256MB Cache for RAID Network Attached Storage, Data Recovery Service – Frustration Free Packaging (ST14000NEZ008)	14 TB	$619.99	$44.29	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf Pro ST12000NE0008 12 TB Hard Drive - 512E Format - SATA 600-3.5" Drive - Internal - 7200RPM - 256 MB Buffer - Hot Pluggable	12 TB	$534.99	$44.58	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf 16TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 6GB/S 7200 RPM 256MB Cache for Raid Network Attached Storage, with Rescue Service (ST16000VN001)	16 TB	$713.45	$44.59	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 16TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD161KFGXSold by Amazon	16 TB	$714.99	$44.69	NAS HDD · SATACMR	Prime	Buy on Amazon
Seagate IronWolf 10Tb NAS Internal Hard Drive HDD – 3.5 Inch SATA 6GB/S 7200 RPM 256MB Cache for Raid Network Attached Storage (ST10000VN0004)	10 TB	$449.99	$45.00	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf 10TB NAS Internal Hard Drive, 3.5" SATA 6Gb/s, 7200 RPM, CMR, RAID Optimized, IronWolf Health Management – for NAS Systems (ST10000VNZ00)	10 TB	$449.99	$45.00	NAS HDD · SATACMR	-	Buy on Amazon
Seagate Exos X14 ST12000NM0008 12 TB Hard Drive - 3.5" Internal - SATA (SATA/600)	12 TB	$540.38	$45.03	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
HGST - WD Ultrastar DC HC520 HDD \| HUH721212ALE600 \| 12TB 7.2K SATA 6Gb/s 256MB Cache 3.5-Inch Helium Data Center Internal Hard Disk Drive	12 TB	$541.45	$45.12	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 12TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD122KFBXSold by Amazon	12 TB	$541.68	$45.14	NAS HDD · SATACMR	Prime	Buy on Amazon
Western Digital 10TB WD Red Plus NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 GB/s, CMR, 512 MB Cache, 3.5" - WD100EFGXSold by Amazon	10 TB	$455.75	$45.58	NAS HDD · SATACMR	Prime	Buy on Amazon
Seagate IronWolf Pro ST12000NE0008 12 TB Hard Drive - 3.5" Internal - SATA (SATA/600)	12 TB	$550.00	$45.83	NAS HDD · SATACMR	-	Buy on Amazon
Seagate IronWolf Pro 16TB 7200 RPM SATA 6Gb/s 3.5" NAS HDD (ST16000NE000)	16 TB	$735.32	$45.96	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital Ultrastar DC HC550 WUH721816ALE6L4 0F38462 16TB 7.2K RPM SATA 6Gb/s 512e 512MB 3.5" SE HDD	16 TB	$744.45	$46.53	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 16TB WD Gold Enterprise Class Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, 512 MB Cache, 3.5" - WD161KRYZSold by Amazon	16 TB	$749.00	$46.81	Enterprise HDD · SATACMREnterprise	Prime	Buy on Amazon
WD WUS721010ALE6L4 Ultrastar DC HC330 0B42266 10TB 7200 RPM SATA 6Gb/s 256MB Cache 3.5-Inch Enterprise Hard Drive	10 TB	$471.45	$47.15	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Toshiba MG09ACA18TE 18 TB Hard Drive - 3.5 Internal - SATA [SATA/600]	18 TB	$849.99	$47.22	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Constellation ES.3 ST4000NM0033 4 TB Hard Drive - 3.5 Internal - SATA	4 TB	$189.27	$47.32	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Exos X24 3.5" Internal Hard Drive - 12TB, Single, SATA III 6 Gb/s	12 TB	$569.99	$47.50	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Exos X20 ST20000NM007D 20TB 7.2K RPM SATA 6Gb/s 3.5in Hard Drive	20 TB	$950.00	$47.50	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro, 14 TB, Enterprise NAS Internal HDD –CMR 3.5 Inch, SATA 6 Gb/s, 7,200 RPM, 256 MB Cache for RAID Network Attached Storage (ST14000NT001)	14 TB	$667.00	$47.64	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 20TB WD Gold Enterprise Class SATA Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, 512 MB Cache, 3.5" - WD203KRYZ	20 TB	$953.99	$47.70	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 8TB WD Gold Enterprise Class Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, 256 MB Cache, 3.5" - WD8005FRYZSold by Amazon	8 TB	$387.99	$48.50	Enterprise HDD · SATACMREnterprise	Prime	Buy on Amazon
Western Digital 22TB WD Gold Enterprise Class SATA Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, 512 MB Cache, 3.5" - WD221KRYZ	22 TB	$1,067.31	$48.51	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf Pro 12TB NAS Internal Hard Drive HDD – 3.5 Inch SATA 6Gb/s 7200 RPM 256MB Cache for RAID Network Attached Storage Data Recovery Service – Frustration Free Packaging (ST12000NEZ008)	12 TB	$584.45	$48.70	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital WUH721816ALE6L4 3.5" 16TB SATA 6Gb/s7.2K RPM 512M 0F38462 512e/4Kn HDD	16 TB	$784.45	$49.03	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf 18TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 6Gb/s 7200 RPM 256MB Cache for RAID Network Attached Storage, Rescue Services – Frustration Free Packaging (ST18000VNZ00)	18 TB	$884.45	$49.14	NAS HDD · SATACMR	-	Buy on Amazon
Seagate Exos X18 ST18000NM001J 18 TB Hard Drive - Internal - SATA (SATA/600)	18 TB	$884.45	$49.14	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Western Digital 12TB WD Gold Enterprise Class Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, 512 MB Cache, 3.5" - WD122KRYZSold by Amazon	12 TB	$594.99	$49.58	Enterprise HDD · SATACMREnterprise	Prime	Buy on Amazon
WD Red Pro 18TB NAS 3.5" Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, CMR, 512MB Cache	18 TB	$894.45	$49.69	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital Ultrastar WUH722020CLE6L4 20 TB Hard Drive - 3.5" Internal - SATA - Energy-Assisted Magnetic Recording (EAMR) Method	20 TB	$999.00	$49.95	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Toshiba 4TB Enterprise Internal Hard Drive – MG Series 3.5" SATA HDD for Server, Storage, 24/7 Operation, Hyperscale, Cloud (MG04ACA400E)	4 TB	$199.99	$50.00	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate Exos 7E8 4TB 512n SATA 128MB Cache 3.5-Inch Enterprise Hard Drive (ST4000NM0035), Mechanical Hard Disk	4 TB	$200.00	$50.00	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate 8TB Enterprise Capacity 3.5 HDD 7200RPM SATA 6Gbps 256 MB Cache Internal Bare Drive (ST8000NM0055)	8 TB	$400.00	$50.00	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon
Seagate IronWolf 10TB NAS Internal Hard Drive HDD – 3.5 Inch SATA 6Gb/s 7200 RPM 256MB Cache RAID Network Attached Storage Home Servers - (ST10000VN0008)	10 TB	$500.00	$50.00	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 14TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" -WD142KFGXSold by Amazon	14 TB	$701.45	$50.10	NAS HDD · SATACMR	Prime	Buy on Amazon
Western Digital WD Red Plus Internal Hard Drive 8TB CMR 3.5 inch SATA 5640 RPM 256MB NAS WD80EFPX-AJP Eco Package	8 TB	$405.08	$50.64	NAS HDD · SATACMR	-	Buy on Amazon
Western Digital 18TB WD Gold Enterprise Class Internal Hard Drive - 7200 RPM Class, SATA 6 Gb/s, 512 MB Cache, 3.5" - WD181KRYZ	18 TB	$924.30	$51.35	Enterprise HDD · SATACMREnterprise	-	Buy on Amazon

Frequently asked questions

How much storage can the DS225+ hold?

48TB raw with two 24TB drives, which is 24TB in SHR, one drive of protection. It lists no expansion unit, so the two bays are the ceiling.

How much storage can the DS725+ hold?

48TB raw with two 24TB drives, which is 24TB in SHR, one drive of protection. Beyond that, one DX525 expansion unit takes it to 7 bays, which is 168TB raw.

Which RAID should I use on the DS225+ and DS725+?

SHR, one drive of protection. With two bays there is one sensible protected layout: both drives hold the same data, so you keep one drive's capacity and survive one failure.

Does the volume limit on the DS225+ and DS725+ affect large drives?

A full set of 24TB drives in SHR gives 24TB, which is inside the 108TB single-volume limit on both models, so the limit never constrains the internal bays.

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