2 drives - same files - different "Used Space"

[spandexninja]

I have 2 identical external hard drives (Western Digital 2tb My Passport) and I copied all the data from 1 drive to the other. When I check the properties of the files the sizes match, but when I check the properties of the drives the used space differs by ~4mb. I've been checking the MD5 hash data of my bigger files and so far there are no differences. uTorrent's hash check (force recheck) also says the files are ok.

Here is a picture of my problem: http://i.imgur.com/btoFWII.png

-both drives are formatted the same (NTSF, same cluster size)
-Crystal Disk Info and Windows Error Check both say the drives are fine
-There were no vibrations that might disrupt the drives when copying. I wasn't even using the computer.
-There are no hidden files in either drive

Can anyone explain this 4mb size difference?

 

[fzabkar]

Try CHKDSK in readonly mode.

AIUI, an NTFS file system records the free and in-use clusters in a $BITMAP metafile. The $MFT also records the cluster usage for each file on the volume. I would think that CHKDSK would walk down the entire directory tree and compare the total number of in-use clusters reported by the $MFT against the number recorded in the $BITMAP.