There's been some fairly extensive discussion of this over on the xicons forums... here's the gist:
The hard drive icons can't be changed in the release build, and it's a bug that has been reported to Apple but not yet fixed. A custom icon can be assigned to a drive, but X will not display it. The workaround for the time being is to use aliases instead of the actual drives on your desktop. Of course, making an alias of the drive won't work (it'll just show the generic icon that can't be changed), so here's what you do...
1. Make an alias of a file on your hard drive (any old file will do).
2. Use the inspector to change the alias' icon
3. Use the inspector to retarget the alias to one of your partitions.
Once you've made an alias for each drive, you can turn off the "show disks on desktop" option and get the general workaround 'til this issue is fixed
If you go to http://www.xicons.com and look at the custimization article you can change HD icons. This is mdifying the default icon. With those instructions you can change the Trash new folders and bunch of other icons. Now the only problem is you can only pick one hard drive icon. So multiple partitions wil have the same custom icon. I found the instructions to be straightforward and simple enough.
Worth the effort if you want to modify the majority of the system icons.