Not sure if you have fixed this problem or not but I was having the same problems and here is how I fixed it. Go to /System/Library/CoreServices/ManagedClient.app/Contents/Resources and look for MCXDiskSetup. You can either add a .old to the end of it or delete the file. I prefer to add the .old to the end because I dislike deleting system files. You will have to do this on all machines you are having problems with. I hope this helps.
This was driving me mad for a few hours, then I found a Managed Client setting that fixed it.
Under MEDIA ACCESS in the OTHER MEDIA tab, the box for REQUIRE AUTHENTICATION was ticked against the INTERNAL DISKS
Once I removed this (in fact stopped managing it altogether) the annoying messages stopped.
Hope this helps someone else in future. Googling the message only gave me 4 results!! Getting support for Apple related enterprise level problems is extremely frustrating.