It sounds like you will be changing your WL-HDD beyond all recognition, and I doubt that many other people (if any) have attempted what you are doing. This means you won't get many replies just because other members really don't know.
Anyway, you can't fry a unit by having a power supply that provides more current than you need. The WL-HDD will only use what it requires.
You can fry the unit by using too large a voltage or by connecting the power rails the wrong way round.
There are a few threads in the Custom Development and Pics and Specs sections to do with adding hard discs etc.
I hope you are good with a soldering iron as this is also a quick way to break your unit.
Ade