Unable to 'empty' flash drive?

So, I'm having a rather interesting problem. I've had this particular flash drive for a long time now, and I've never had any problems. Generally, it's been confined to a Windows environment, but since I've been trying to use it with my Mac lately, it keeps running out of space when it shouldn't be.

For example, it's a 4 GB card, and 800MB of information is filling it up... I tried formatting it with Disk Utility, and this fixed things - until I moved some files back on to it. After the initial file transfer, everything else says the drive is full. Deleting all the items from it doesn't help.

Anyone got any idea what might be causing this?