You have a disk error on the drive. You need to do one of several things depending on what you want and if you have backups or not.
First off backup the system, which can get tricky if the drive has a error, is the only drive in the system and is not allowing you to write to disk.
Once you have everything you can backed up (you want to do this in case things go bad and it can if the damage is bad), then go into single user mode, unmount (umount command) the drive and check it (use the fsck program).
I’ll bet if you reboot the system and watch it it’s prompting you about the problem and whoever skipped over it, and/or it had a timeout and rolled past.
if the drive is write protected yes. But I can’t be sure as you’ve not provided enough info.
at a linux prompt type mount
it will output all the drives that are mounted and how. At the end of each line will be some codes (rw) means what is allowe, in this case read write. If the drive in question is listed as r only that would do it.
Recordings are placed at /var/spool/asterisk/voicemail/… by default so look for a drive that contains all or part of that path. Note that / is root and it can contain that if you don’t find a /var anyplace.