Following the instructions on DroboShare, the Drobo disk must be reformatted as "MacOS Extended", i.e. not journalled. Furthermore, to prevent Time Machine from filling up the Drobo indefinitely, one may like to partitioned it:

Partition 1: Tardis (500GB) ... for Time Machine Partition 2: Dalek (1.5TB) ... for general use

Next problem was to copy the previous Time Machine image onto the new partition. For better performance, we did this with both the old backup disk (Diskus) and the Drobo (Tardis & Dalek) connected. This suggestion from an arstechnica forum appeared to do the trick:

sudo /usr/local/rsync/bin/rsync -aNHAXx --protect-args --fileflags \
    --force-change /Volumes/Diskus/Backups.backupdb /Volumes/Tardis

NB. that this is not the rsync that came with Mac OS X, but version 3.0.1 that we have compiled with support for extended attributes.