I have a wd caviar black 500 gig and another 1tb. When installing win 7 and then arma 2 and its updates, the 500 gig shows 0% fragmentation after defrag but the 1tb shows 1% after defrag. This is after multiple fresh installs on each disk. Two 1tb wd caviar blacks have done the same thing with the same exact data and game on them, again the 500 gig doesn't do that. What's with that? Is it due to dual platter design or something? I know it may not be a big issue, but i have 30 days on the return and would rather know if its normal or needs to be replaced.
Fragmentation occurs at the filesystem (logical disk) level, so the physical storage hardware does not play a role.
1% is nothing to worry about. My guess is that it's just a reporting error by the Windows defragger. It does not report or defrag larger fragments (64MB or bigger, supposedly there's no benefit from defragging these) by default which is just stupid if you ask me. It also does not defrag some system files.
Just download the free evaluation version of a good third party defrag utility and analyze with it. It will also show you the disk map. Usually no two defraggers will give the exact same analysis because each one differs in it's interpretation of what's fragmented, but the numbers should be similar.
To compare the pre and post-defragmentation results across the drives, your start condition has to be exactly the same i.e. exactly the same disk image should be installed for all the drives and defragged to compare.
But anyway, if you have time, check with a reliable utility to compare. It may probably give the same analysis for all the drives.
if you're unsure about the condition of the drives itself, then maybe a good idea to run diagnostics and also some HDD benches (HDtune etc which measures raw disk performance, not filesystem performance so is independent of fragmentation).