Drobo failing to mount a partition

I use a Drobo as a source for a Plex server. Having my children get video from Plex is much neater than having them ransack a set of DVDs.

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.