I'm new here, because I have a new customer with Symantec Backup Exec 2014 running. He backups to two NAS. One NAS is always connected for the daily backup. The other one is stored out of the office and connected once a month for offsite backup. Everytime he now connects the offsite NAS to the network, he get a write protected medium. I think, this post is very similar to mine issue, but the solution link is not working anymore:
I'm very happy about every input :)
By design Backup Exec will mark as read only any disk device that is disconnected for more than 2 weeks (for devices seen as fixed disks including USB and NAS) and for 4 weeks (for Cartidge/RDX style devices)
This is because usually when a device is disconnected for that long, it is against long term storage and has been reconnected for a restore and this is needed to stop data being erased by DLM if you have reterned a long time disconected device for a restore where the expiry dates may have been exceeded before the device is reconnected.
If your rotation strategy is longer than these periods you can change the timings - go to the storage section of the Backup Exec general settings (and make sure you scroll the section to see all the options)