Request: Small app that shows when data is being written to Flash drive

Once in a while when I try to run Firefox from my flash drive, it runs really slowly. Other times, it runs fine. I've started to doubt that this is always due to slow read/write tasks on the flash drive. What I would like is a small app that sits in the system tray and shows when data is being written to the flash drive, in real-time. It could be as simple as a red/green dot.

If something like this already exists, please point me in the right direction.