Getting Time Machine back up to a Sparsebundle on external HDD (Drobo)

Hey,

I've been chasing this issue around for a while but can't seem to find the answer.

There are a lot of resources out there describing how to get your Mac to backup to a Sparsebundle on a NAS, and more resources telling you how to write protect the Info.plist file so that Mac OS doesn't resize the image to take up all of your NAS.

However I've got a Drobo connected directly to my Mac Mini. The Drobo pretends to have a 16TB drive so that you can add storage as and when you need it.

I am already using it to back up my MBP over the network to a Sparsebundle and until recently I was using the same scheme (a size limited sparsebundle) to back up the Mac Mini that the Drobo is connected to.

I achieved this by following the excellent directions on http://anitechtalk.wordpress.com/2010/07/19/time-machine-dns323/ (one of the only sites that mentions the new way that Apple handles Time Machine Sparsebundles)

However this has recently stopped working and no matter what I do I still cannot get my Mac to see the Sparsebundle as a TimeMachine aware 'drive'- instead it just backs up to the root of the '16TB' Drobo- and at some point it's going to get rather full.

Argh.

(And yes, I know that the other solution is to re-partition my Drobo to have a small partition at the start of the disk for Time Machine to use- but it's too late for that as I've already got 1.85TB of data on there and I doubt that I can re-partition that without losing all of the data.)

Can anyone help me get Time Machine to accept my Sparsebundle as a 'disk' that it can backup to?

Thanks in advance.