Same questions apply. I tried looking up your drive based on what you said, but there are about 9 models of the ACOMData 500GB external drive (USB, F/W, 'Hybrid', etc). Not knowing which you have makes it a little tougher.
That said, here's some basics to know (some are BASIC, but I have to say them):
If the drive can take an external power source/has a power switch it needs the external power. There are bus-powered drives out there (e.g. WD Passport drives).
If you're still uncertain, the size usually gives it away. Small (2.5" drives) are bus-powered. Big (3.5" drives) are not.
Bottom-line, no power, no drive.
Two, Leopard came with a bunch of new drivers. Some supercede the versions shipped with Tiger+ driver packages shipped with devices/available on manu. website. Here's a good rule of thumb:
Try your device before installing
ANY software from the manufacturer. You may be surprised to see it work just fine.
What if you installed first? well, if the device is working, who cares? If it's not, you'll need to revert back to the old drivers. You can do this through Time Machine, or reinstalling driver packs from 10.5.1 or the Leopard install disk.
If you're still stumped, drive stuff like this is solved WAY faster in-person. We're talking hours, not days. Be it Genius bar or some local geek you know, give in-person a shot. Offer your geek friend food/beer and let them at least diagnose what's going on. They may tell you a better way to phrase questions to the forum/Apple support/ACOM support.
Finally, don't be afraid to try support from ACOM. You paid for it in the cost of the drive. Might as well.
None of those are silver bullets, but I hope they help.
