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Copy the icon (Option-C).

Right-click on the HDD and choose "Get Info".

Click on the HDD icon in Get Info.

Paste the icon (Option-P).

This works with OS X and FAT file systems. It does not work with NTFS as the default Apple driver is read-only. If you use a third-party driver that allows you to write to NTFS, then it works there, as well.
 
i think candybar is supposed to do all of that kind of stuff. i haven't actually used it yet though. fraid i'll mess something up :( haha
 
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