Not MoD's fault
Look people, stop blaming the MoD for this data loss. The hard drive was held on a secure site of a private company.
There are a whole load of regulations around storing such data and security records will have to be kept by the private company which are available for inspection by the MoD. The MoD have almost certainly discharged their obligations correctly, the problem lies with some individual within the private company.
At the end of the day, it could be as simple as an employee of EDS not returning the removable hard drive back to the safe at the end of the day, being left out on a desk somewhere.
That's not the MoD's fault.
It could have been misplaced, or it could have been intentionally removed from the building.
The only way you're going to stop that is physically search everybody every time they come and go from the building (or fasten RFID tags on all the drives and have scanners at the entrances/exits to the building).
This is not the same kind of case of taking USB flash drives, sending unencrypted CDs through the post. It's almost certain the hard drive is not routinely taken off site: it's not intended for that.