write protect error suddenly preventing backups

[untorqued]

I'm using BE with an iomega REV drive. I've had an automated full backup going on twice a week for close to a year. About two weeks ago, backups started failing with the message

"The job failed with the following error: The media is write protected."

I had two-day write protection enabled by default, but even using cartridges that I hadn't used in a while triggered this message.

I've now totally turned off write-protection (and there are no physical write protect tabs on these cartridges). I've also updated to the most recent version of BE, my Windows 2k3 server is up to date on patches.

I'm beginning to suspect that the error is a red herring, and that something else must be going on, but I'm not sure just where to look. 

[untorqued · Accepted solution]

I spun my wheels on this for a day and a half, but finally seem to have solved it.

I noticed the disks I was having trouble with were totally full (0 B free reported by Windows). I was unable to format them using Windows Explorer (got a seemingly nonsensical write protection error).

I installed the latest REV system software for Windows, and was then able to format one of the disks in Windows Explorer. The backup job is now running again!