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.
9/3/10 Ebay has them 12v 3 amp. Cost is around $13. The bad news is they are shipped from Hong Kong. Domestic supplies cost around $19. I am thinking about ordering one myself to replace the one I lost. My unit called for a 12v 2 amp or at least that is what it calls for on the case. When I researched the other listings on Ebay, they are all 12v 3amp supplies. They are all called replacements supplies because Maxtor was purchased by Seagate and no longer manufactures the origional replacement par
Do a search on Ebay using the words MAXTOR POWER SUPPLY.