For if your Linux box has some drives that normally automatically mount fine, but suddenly one or more are in read-only mode.
This can happen because a common setting in fstab is “errors=remount-ro” which means that if there are any errors while mounting, remount it in read-only mode. Which is to help prevent data-loss.
To (try to) fix this, first, unmount the drive. Either with umount or by commenting out the appropriate line in
Find your disk:
Run some checks on the drive:
$ sudo fsck -f /dev/sdb1
and (if you have a lot of time)
$ sudo badblocks -nsv -b 4096 -c 65536 /dev/sdb
Hopefully those run smoothly.
Remount the drive, possibly by uncommenting the appropriate line in fstab and rebooting again.
Confirm the drive is mounted and writable.