Which is better for a NAS?

The Exos M (B) is the better pick for an always-on NAS because its 3TB platter density is designed for lower power draw per terabyte. Both are 7200 rpm enterprise drives with the same 28TB capacity and SATA interface.

Is the Exos M newer than the Exos X28?

Yes. The Exos M is Seagate's newer platform built on 3TB platters. The Exos X28 is the prior-generation model, though both deliver 28TB at 190 MB/s.

Should I worry about recertified drives?

Recertified drives are factory-refurbished and can perform like new, but they carry shorter warranties and unknown prior hours. Confirm the condition and warranty in writing before you buy either model.

Do they perform the same?

On paper yes: both are 7200 rpm SATA 6Gb/s drives rated at 190 MB/s read. Real-world sequential transfers will be close, so pick on condition, warranty and price rather than speed.

Neither listing gives workload or MTBF figures, so longevity depends on condition and use. A new-in-box unit with a full warranty is the safer bet over a recertified one for long-term reliability.

Is one worth extra money?

Pay more only for a new-in-box drive with a full warranty. If the Exos M is recertified and the Exos X28 is new, the X28 is the better value at a similar price.

Seagate Exos X28 28TB SATA Enterprise Hard Drive (ST28000NM000C) vs Seagate Exos M ST28000NM003K 28TB SATA 6Gb/s 7200RPM Internal Hard Drive

Both are 28TB Seagate enterprise drives at 7200 rpm with 190 MB/s read speed, so the choice comes down to model generation and condition. The Exos M (B) uses 3TB platter density for better power efficiency and is the newer design. The Exos X28 (A) is the prior-generation model. Check the exact condition and warranty of each listing before buying, since recertified units appear in both lines.

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