I've an external hard drive that i use to transfer files between windows-based applications (hardware) to GNU/Linux Debian for processing. I recently free'd a big chunk of the space. However, when I plug the drive in the GNU/Linux machine it fails to report the correct free space (and therefore, cannot copy onto it). I've checked the drive on other windows machines and the free space reported is correct.
Any clues on how I can get correct free memory statistics?
Thank you.

What Linux tools are you using? e.g. What does "df" tell you?

Also, do "ls -al" to list all hidden files. Is there maybe a folder called "trash"?

I did check for the folder "trash" and made sure it's empty. There are no other hidden files. The drive is an FAT type specifically a data drive for transferring image files after recording to the linux machine for analysis/processing. df tells me I've approx. 2.4 GB free space whereas in windows I'm getting >7GB free space. I've tried rebooting but to no avail.The memory was free'd in windows and I did empty the recycle bin (which led to a gain of almost 5GB that I can't seem to locate in GNU/linux). I can copy an image file within windows whereas in GNU/Linux I cannot because of insufficient space.

re-booting is not going to change how Linux sees this drive.

If ls -al shows no hidden files, my next step would be "du". This gives you the disk space used by each directory and file. "man du" for options

Just another thought if were are talking about an ntfs partition: did you check its .Trash folder?