Using a flashdrive?

Wasn't sure where exactly to post this.... but here goes

I have 128mb flashdrive and I have say 80mb on it. I want to free up space...so I delete all the 80mb.

but on my mac (macbook pro) it shows that the flashdrive only has 48mb free STILL even after I delete EVERTYHING

Why is this? Can someone please help

I realize that on a PC you can choose folder options and do something on the lines of "show hidden files" which on a flashdrive will allow you to delete hidden files that are taking up space. But on a mac how do you do this....or at least just get back to 128 mb free of space

thanks