Seems like a largish discrepancy. It can be explained (at least partially, or perhaps fully) by:

1. There's a virus / trojan hiding in there somewhere. or..

2. The drive with less "free space" used to have really big directories, and now files have been deleted from those, but not all files. So the directories are still huge, but sparsely populated. This uses "invisible" disk space like what you report.

Cheers