· Atomic Racks
Seagate Exos X16 vs X18: Which to Buy Used
Transcript
Buy whichever Exos generation gives you the lower price per terabyte at the capacity you want. In a NAS, the X16 and X18 are interchangeable. Both share the same 7200 rpm helium platform, 256 megabyte cache, 550 terabyte per year workload rating and five-year original warranty. The X18 adds an 18 terabyte top capacity and slightly higher areal density. On paper the X18 is marginally faster, at around 270 megabytes per second sustained versus around 260 for the X16. In an array you will not notice it, and power draw at the same capacity is within a watt. To read the part numbers, the G suffix family is X16 and the J suffix family is X18. Within each, codes with zero zero one or zero zero zero are SATA, and zero zero two or zero zero four are SAS. What actually matters is capacity per bay, since 18 terabytes means X18, and the age of the used pool, since X16 pulls tend to have more hours. Compare hours and condition, not generation. The full guide and our per-unit test reports are at atomicracks dot com.
Buy whichever Exos generation gives you the lower price per terabyte at the capacity you want. The X16 and X18 share the same 7200 rpm helium platform, 256 MB cache, 550 TB/year workload rating and five-year original warranty. The X18 adds an 18 TB top capacity and slightly higher areal density; in a NAS the two are interchangeable.
What the two generations are
Seagate names Exos capacity-optimized drives after the largest capacity in the generation. The Exos X16 family topped out at 16 TB and shipped in 10, 12, 14 and 16 TB versions. The Exos X18 family raised the ceiling to 18 TB and was offered in 10 through 18 TB. Both are helium-sealed, both spin at 7200 rpm, and both come in SATA 6 Gb/s and SAS 12 Gb/s versions.
The X18 reached 18 TB by increasing areal density rather than adding platters, so a 16 TB X18 and a 16 TB X16 have similar mechanical layouts. Seagate's own figures put the X16 at around 2.5 million hours MTBF and the X18 at the same, with both rated for 550 TB per year of workload, which is the enterprise capacity standard across the industry.
Spec comparison
| Specification | Exos X16 | Exos X18 |
|---|---|---|
| Capacities | 10, 12, 14, 16 TB | 10, 12, 14, 16, 18 TB |
| Spindle speed | 7200 rpm | 7200 rpm |
| Cache | 256 MB | 256 MB |
| Fill | Helium | Helium |
| Recording | CMR | CMR |
| Interfaces | SATA 6 Gb/s, SAS 12 Gb/s | SATA 6 Gb/s, SAS 12 Gb/s |
| Sector formats | 512e and 4Kn | 512e and 4Kn |
| Max sustained transfer | Around 260 MB/s | Around 270 MB/s |
| Workload rating | 550 TB/year | 550 TB/year |
| Original warranty | 5 years | 5 years |
The transfer figures are datasheet maximums for the largest capacity in each family, measured on the outer tracks. Smaller capacities in each line are a little slower, and every drive slows toward its inner tracks.
Model numbers to recognize
Seagate part numbers encode capacity and interface. The first group is capacity in GB; the suffix tells you the generation and interface.
- ST16000NM001G: Exos X16 16 TB, SATA, 512e. The most common X16 on the used market.
- ST16000NM002G: Exos X16 16 TB, SAS 12 Gb/s, 512e.
- ST14000NM001G: Exos X16 14 TB, SATA, 512e.
- ST10000NM002G: Exos X16 10 TB, SAS.
- ST18000NM000J: Exos X18 18 TB, SATA, 512e.
- ST16000NM000J: Exos X18 16 TB, SATA, 512e.
The pattern to remember: the G suffix family is X16 and the J suffix family is X18. Within each, 001 and 000 denote SATA 512e and 002 or 004 denote SAS. Seagate also sells SED (self-encrypting) and 4Kn variants with different trailing codes, so verify the exact part number against the Seagate product page if you need a specific sector format.
SATA or SAS variant
The mechanical drive is the same; only the interface board and firmware differ. SAS versions support dual-port connections, a 12 Gb/s link, and the richer SCSI command set, none of which matter in a single-host NAS. What does matter is that SAS drives need an HBA, and that used SAS drives sometimes arrive formatted to 520 or 528 byte sectors from storage arrays. Used SAS pricing is usually lower per terabyte because the buyer pool is smaller. If you already run an HBA, the SAS variant is often the better deal. See SAS vs SATA for a homelab NAS.
Power and heat
Both families idle around 5 W and draw roughly 9 to 10 W under random read and write load, according to Seagate's datasheets. The difference between generations at the same capacity is within a watt. Helium fill is what keeps these numbers low compared with air-filled drives of similar capacity, since the thinner gas reduces drag on the platters. In an eight-drive array the generation choice changes your power bill by a negligible amount.
What differs in practice
In a RAID or ZFS array the two generations behave the same. Resilver times, scrub throughput and random I/O are within the margin of drive-to-drive variation. The decisions that actually affect your build are these.
- Capacity per bay. If you are bay-limited, the 18 TB X18 is the reason to pick that family. If 16 TB per bay is enough, the X16 is usually cheaper.
- Age of the used pool. X16 drives started shipping in 2019 and X18 in 2020, so X16 datacenter pulls tend to have more hours. Compare power-on hours and tier rather than assuming the newer family is fresher.
- Replacement planning. If you are expanding an existing array, matching the capacity matters more than matching the generation. A 16 TB X18 is a fine replacement for a 16 TB X16.
- Firmware. Both families have had firmware updates. Check the version on arrival and update if Seagate lists a newer one for your part number.
Buying used: what to check
Whichever family you choose, the condition checks are identical. Zero reallocated, pending and uncorrectable sectors on SATA (or zero grown defects on SAS), a completed extended self-test, and a sustained read benchmark close to the datasheet figure. Helium level, SMART attribute 22, should read 100. Our guide to what fails a drive covers the thresholds. Browse current Seagate Exos stock across both families.
FAQ
Is the Exos X18 faster than the X16?
Marginally, on paper. The higher areal density moves a little more data per revolution, which is where the roughly 10 MB/s difference in sustained transfer comes from. In an array you will not notice it.
Can I mix X16 and X18 drives in one array?
Yes, as long as capacities match within a vdev or RAID group. Mixed generations are common and arguably good practice.
Which has the better reliability record?
Both are mainstream datacenter drives with similar published ratings. Public fleet data shows low failure rates for both families, with variation between capacities and batches that is larger than the difference between families. Buy on condition, not generation.
Are these drives loud?
They are enterprise drives with 7200 rpm spindles and firm actuators. Seek noise is audible in a quiet room. Helium drives are quieter at idle than air-filled drives of the same class, but none of these are living-room drives.
What we do at Atomic Racks
Every used drive we sell gets a SMART extended self-test, a fio benchmark and NIST SP 800-88 sanitization with read-back verification. Hours are read from the drive and never reset, and each unit is priced into a power-on-hour tier from Open Box (under 100 h) to Value (over 40,000 h). The QR tamper seal on each certified unit opens its public test report. 90-day warranty on standard items, up to 2 years on certified tiers, 30-day returns and free shipping to the lower 48. Details on how we test.
Further reading: Buying matched drive lots for RAID.