The problem with these hard drives are their size. You will have a really hard time trying to find another drive. As for the dimensions, I don't know exactly which size will fit the 20GB 3G. It may be that the 40GB is now the same size as the 20 use to be. But I am doubting it. Check the dimensions on the new ipod 40GB and your 20. These drives arn't cheap either, by any means. They are probably really hard to find as well.
As for the back plate. I have never taken apart an ipod personally so I wouldn't know if its possible. But, again you need to make sure the dimensions are the same. Also, finding a back from one of these ipods might not be the easiest task either.
Best of Luck, let us know how it goes.
"Fix it 'til it Breaks."
Jacob - EiC & Director of Technology Mac Pro Quad 2.66 - 4GB RAM 160 GB SATA RAID 1 - 650 GB Storage Quad 19" Widescreen LCDs Accessorized to the Hilt
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