Time Machine verification issue

Hello. I recently purchased a Drobo (1st gen, off ebay) to replace a 1TB WD MyBook I was using as a Time Machine backup and Media Server.

The Drobo is connected to an Airport Extreme base station over USB 2.0. My Mac is connected by ethernet to the Airport Extreme.

I followed the instructions on the Drobo website to create a separate 500 gb partition for Time Machine and the rest to accommodate my media collection. http://support.datarobotics.com/app/answers/detail/a_id/67/related/1/session/L2F2LzEvc2lkLzdta1VHNmZr

The Time Machine backup seems to go OK, taking several hours to backup 160gb of data. Later on in the day, I’ll get a message that says:
“Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you.”

I’ve tried redoing the backup 4 or 5 times now. I’ve tried repairing permissions and the repairing the disk on both the Mac and the Drobo. I’ve learned that Time Machine Backups to Airdisks aren’t officially supported by Apple (Although I never had a problem with my previous hard drive attached to the Airport Extreme, and Drobo say it’s fine on their website - “Time Machine with a Drobo device connected to an AirPort Extreme or network attached (NAS) works. Just make sure you have updated Time Capsule or Airport Extreme to firmware 7.3.1 or later.”

Every time, after a couple of hours, I get the verification warning and have to delete the backup and start again. I wouldn’t mind just going and buying another 500gb external drive to just attach to my Mac, only I now have a separate 500gb volume sitting on my Drobo that I can’t do anything with and I can’t delete because I don’t have anywhere else to transfer the data from my Drobo while I reformat it!

I’m running out of ideas and would love it if someone can offer some advice. Thanks guys!