odd transfer error.
I've got 15.1 GB on my external hardrive
and am trying to transfer a folder which is 4.2 GB from my computer's hardrive.
however apparently my external hardrive does not have the required space.
I've diskcleanup, and attempted on compressing, however the compression led it to not extract correctly.

The external drive is most likely formatted to FAT32.

FAT32 cannot handle files larger than 4GB. You are just over the limit.

You can either breakup that folder or change the external drive to NTFS.

This is Microsoft's page on the matter:

http://support.microsoft.com/kb/307881