A message on a Seagate forum makes it sound like the thing takes 12 volts at 2 amps.
But, another messageboard suggests that you need 5 volts at 2 amps.
To start out, try plugging all of your “wall warts” into the drive and ignore all of the ones that don’t fit.
Next, look for any that say they put out 5 volts at 2 amps, or 12 volts at 2 amps, to narrow down the herd. If you’re lucky, you’ll have just one that meets either of those specs, and that should be the one you want. The difference between 5 and 12 volts is substantial to computer hardware, so if you have a 5 volt and a 12 volt option, and they’re the same polarity, try the 5 volt first.
Another option would be to visit your local computer hut and if you ask nicely, they might let you open one up (if the boxes aren’t shrink-wrapped) to see what the specs are.
I found a picture of one - got any that look like it?
If all else fails, replacement wall warts are available for $39.
Strangely, it looks like this drive is meant to be used with a “Y” USB cable and powered by the computer, rather than an external power pack. You’re sure this thing has a power supply? 
ETA:
Of all the wall warts I have here, only the ones for my IBM Thinkpad or Apple MacBook have the maker’s name on it, or are at least physically distinctive. Everything else is some generic Korean or Taiwanese maker’s name.
Silver Sharpies are wonderful for tagging the things when you get them. The ink won’t pop off like a Dymo tag might.