The "reserved for expansion" space is effectively dead. Drobo likes its drives to be of a similar capacity. If they're not, the result is "unusable space... because there is not enough capacity to protect data in this space from a single-drive failure". The reserved area is essentially the difference between the biggest hard drive and the next one down. Insert two 80GB hard drives and there's almost no reserved space. Bung in a 100GB unit and 18GB are lost; replace the 100GB drive with a 160GB model and you lose 77GB. So keep the drive sizes close, OK?

And you'll note that that's "single-drive failure" not "multi-drive failure". Drobo's redundancy only goes so far, but it's within the tolerance of almost all desktop-storage users.

Later, we went back to two 80GB drives and tried to fill them. As the spare capacity dwindled, the Drobo flashed an orange LED at us which shortly went red to indicate it needs a new drive urgently.

Interestingly, at this point, while data was apparently still being copied, the system was barely registering increases in the total amount of information sent to the drive. A slowdown in the remaining time for the copy process feels like the Drobo's trying to compress as much of the incoming data as it can pending the insertion of a fresh HDD.

Danger, Will Robinson

Now, we got tired of waiting for something to happen and decided to put the Drobo out of its misery. We slipped in a 160GB disk - which was formatted by the unit and made available to use in seconds - at which point the time to completion began falling again and the tally of bytes copied over to the drive began to increase at the rate we'd expect it to run at. It looks like Data Robotics has at least attempted to cram as much data on the drive as it can in a bid to cater for unattended copies and back-up runs that take the unit over its storage limit.

The purist will complain that there's no option to grab the whole 456.4GB of formatted storage capacity for data, but that's not the point, Data Robotics would say. Drobo's for people who value data safety more than raw capacity. In any case, hard drives are relatively cheap, so if you want more storage space, fit some bigger drives. As we say, it's fully hot-swappable.