I emptied the cruzer 8Gb by deleting the files.

I emptied the cruzer 8Gb by deleting all files. Still it shows 7 GB is present on the usb so I cannot put any new files on it. I am using a mac book pr. I cannot see any files using FINDER. However when I use my other lap top - windows - I see using EXPLORER there are alot of files still remaining, like trashes. I cannot see these using FINDER. How can I make sure these files are deleted from the cruzer using my mac book pro - finder? Thank you much! Elle