The Overdrive is a PCMCIA IDE hard drive controller which connects to the PCMCIA slot of the A1200 and A600. It was usually supplied with a 2.5" 250MB hard drive and is capable of transferring at about 2MB/sec. The unit can be powered by an external power supply to lighten the load on the A600 or A1200's supply. This unit appears to be exactly the same as the Archos Overdrive. Archos may have made the Overdrive under license from Commodore, or vice versa.
Please see the Archos Overdrive.
Thanks to Oliver Brunner