No volume reference numbers found for disks

My Mac has been very unstable since connecting a 5D with repeated crashes.

I’ve run fsck but no problems identified and re-installed the dashboard. It now seems more stable but reading the log files I’m getting a No volume reference numbers found for disk6s entry every few seconds. (Only have two disks in top two bays)

TM: 0x107c13000 12\23 18:11:59 Error - PNPManager::GetFSVolumeRefNum - No volume reference numbers found for disk6s

Is this something I should be worried about and how do I fix this?

If the lights are green and if you could see blue lights on the unit, it is likely your data is still intact, just that it may be inaccessible.

If Disk Utility could not help correct the issue, you could try Disk Warrior, it is a better file system check tool.
http://support.drobo.com/app/answers/detail/a_id/318/