hard disc and memory problem

i have a macbook pro 15inch. and the problem over here is, my external hard disk and pendrive. my external hard disk have the capacity of 320gb. but once i put few application and files in and it take up lets say 100gb. then i shud be left with ard 220gb, but the problem over here is even after i deleted the files and application in my external hard disk, the space left is only 220gb..the same problem goes to my pendrive as well. and i have formated both my external hard disk and pendrive into FAT32 format..can anyone help?

When you empty the Trash (you are emptying the Trash, right?) you must have the external attached or the pen drive attached for the space to be reclaimed.
Emptying the Trash without the drives attached does nothing.

Regards.

thanks so much for the advice cheers