In either Windows or OS X verify the partition information for the drive.
In Windows 7:
Right Click "Computer" and then click on "Manage".
Once the Computer Management window opens select Disk Management from under the "Storage" header.
This should show you the partition information for any connected drives. It will be good to note if it shows the full size of the drive being used.
In OS X:
Open Applications folder in Finder and go to the Utilities directory and open Disk Utility (or press CMD+Space and type in Disk Utility to open it from Spotlight).
Select the USB connected drive and you should see the partition information here as well.
Verifying that the partition information is correct will help determine the next step.
If you are expecting there to be files and they aren't appearing, you can try to use the Windows software Recuva to recover files. If you aren't missing data but just working out a drive space issue, then reformatting/repartitioning the drive should help get your space back to normal. Formatting and partitioning can be done from either of the previously mentioned programs (Computer Management -> Disk Management in Win7/WinXP or Disk Utility in OS X).