The Drobo video partition suddenly decided not to mount on my Mac server. I could see it in disk utility - just couldnt get at any of the files.
The files are all backed up on Backblaze - but getting more than 3TB from Backblaze would not be fun.
After 3 hours with a selection of programmes, I couldnt' get this working... but then discovered this wonderful command:
sudo /System/Library/Filesystems/hfs.fs/Contents/Resources/hfs.util -N /dev/[Disk Device]
Many thanks to Tony Davenport.