Disappearing Freespace On External Hard Drives

Why is it that my 500g external HD now reads "375GB free of 465GB" and my external terabyte HD reads "788GB free of 931GB"? I'm missing 35GB from one drive and almost 70Gb from the other!! Where did it go and how can I get it back??
The solution is probably something simple that I'm overlooking. :confused:
Thanks in advance for any help.

This website is not affiliated, owned, or endorsed by Microsoft Corporation. It is a member of the Microsoft Partner Program.