Presumably an external unit. These are pretty generic so, if it's single drive unit, you could probably just remove the drive and pop it into a replacement case. Twin drive, don't try it as there are RAID issues.
If thats not possible -- research the PSU spec -- probably 5 volt or 12 volt or 12 volt + 5volt. Again pretty generic so sufficient Amp/Wattage and a compatible plug are the only issues -- or you could hard wire in the PSU if you have soldering skills and a multimeter.