Backup Exec 2012 SP3 - USB drives reverting to read-only?

Backup Exec 2012 SP3 on Windows 2008 R2 Hyper-V host

I have 6x 3TB USB drives that are rotated every day. For whatever reason, randomly drives that are plugged in will show up as read-only under the Storage tab. The job will hang and I have to manually set the properties of the drive to "No" under "Limit Backup Exec to read-only operations." Right now it shows 4 of 6 drives in the storage pool as read-only with a little line through a pencil.

Any idea why this happens or how I can prevent it?