I replaced one disk on a mirrored internal boot drive and the system recovered, but the array still shows degraded. Specifically, the internal boot now has three devices instead of two, showing what is presumably the failed drive that I removed/replaced.

The rest of the "data" portion of the pool on the same physical drives appears to have recovered normally, correctly showing the two drives.

What's the correct/safe way to clear this error or update the configuration?

If you have the diags from the replacement would be interested in seeing them, but to fix the issue, open a terminal window and type:

zpool detach flash 14843927469164253652

Thank you, the provided fix resolved the issue.

Diagnostic attached for your review, although I think I rebooted after replacement/rebuild was complete.

diagnostics-20260624-2224.zip

Thanks for the diags. I was able to see the replacement in the syslog-previous and reproduce the issue, it's a corner case that can happen if the missing device has the same identifier as the remaining one.

I created a bug report https://product.unraid.net/b/unraid-os-bugs/p/7-3-1-boot-pool-replacement-can-leave-stale-missing-mirror-member