What drive should I buy to replace a failed drive in SHR?

For an SHR repair, the replacement rule is simple: a CMR NAS-rated drive, at least as large as the drive that failed. SHR is flexible about mixed sizes, so equal-or-larger always works for the repair itself.

Same size or bigger?

Same size is the cheapest fix and restores full protection. A larger drive costs a little more but sets up a future capacity gain: once enough drives in the pool are the bigger size, SHR releases the extra space. Because larger drives are usually cheaper per terabyte, sizing up on a replacement is often the better value.

Why CMR, not SMR

SMR (shingled) drives rewrite overlapping tracks, which collapses their sustained write speed during the heavy, continuous writes of a RAID rebuild. That can turn a rebuild from hours into days, or stall it. Every drive worth buying for a NAS repair is CMR: the table below is filtered to CMR only.

Which families qualify

Seagate IronWolf and IronWolf Pro, WD Red Plus and Red Pro, and Toshiba N300 are the NAS lines, all CMR. Enterprise drives (Seagate Exos, WD Ultrastar) are also CMR and often the lowest price per terabyte if you can accept the extra noise. Any of these makes a valid SHR replacement.