UPDATED: This same option also exists in Backup Exec 2014. These instructions are also the same for 2014. However, in 2014 there is also a checkbox where you can disable this feature. Click here for a screenshot.
I ran across this issue recently when troubleshooting why four removable backup drives in a drive pool kept going into read-only mode at seemingly random intervals. Initially, I would go into the drive properties, switch the drive to allow write operations, only for it go back to read-only just a few weeks later.
My client was using 4 removable USB drives in rotation. These drives were all in one drive pool in Backup Exec. They would swap the drives out weekly, so each drive would be offline for as much as 3 weeks.
What I found, which appears to be new to Backup Exec 2012, is a setting that automatically switches any removable disk to read-only mode after a set number of days. The default is 14 days. I’ve since increased this number past 3 weeks and had no problems.
To fix, select the Backup Exec button in the top left, select Configuration and Settings, and Backup Exec Settings. From the Settings dialog select Storage. Under the Disk Devices section set, the days field higher than your current rotation. For example, 999 days. Click Ok.
I’m still not quite clear what this setting does, and I don’t remember anything like it in 2010 or earlier, but the Help File claims that it:
“Prevents Backup Exec from reclaiming disk space on any disk-based storage device if you re-attach the device after a specified number of days.”
Now, I’m not entirely sure how that would be an issue if your data retention was configured correctly in your job. But modifying that setting to longer than your rotation schedule will fix your issue. Once I find out more about the concepts behind the setting I will post my findings.