If the folder is online but the icon for the folder shows a pencil with a red strike through it, then it is in Read Only mode. (In other words online for restores)
The properties of the drive will show a similar setting that can be changed back to Read write
BE 2012 does not allow multiple Disk storage devices on the same volume (12.5 dis) as such both during an upgrade and ongoing will put a number of disk storage devices into the Read Only mode if it finds a violation of this requuirment.
EDIT: We also put device into Read Only if they have been offline from the media server for too long (default 14 days) reason for this is that sometimes customers will vault USB disks and eventually bring them back for a one off restore. However if DLM reclaim starts against expired media, the sets you need to restore from might get deleted before you can restore unless we marked the drive as Read Only to give you chance to restore from a really old set.