If the metadevice driver takes a metadevice offline due to errors, unmount all file systems on the disk where the failure occurred. Because each disk slice is independent, multiple file systems may be mounted on a single disk. If the metadisk driver has encountered a failure, other slices on the same disk will likely experience failures soon. File systems mounted directly on disk slices do not have the protection of metadisk driver error handling, and leaving such file systems mounted can leave you vulnerable to crashing the system and losing data.
Minimize the amount of time you run with submirrors disabled or offline. During resyncing and online backup intervals, the full protection of mirroring is gone.