Saving files causes diskette drive activity
From MozillaZine Knowledge Base
If you are able to save files to your computer's hard drive but it causes activity in the "Floppy" (A:) diskette drive during the download process:
This can happen if you previously downloaded a file to the diskette drive but the download did not finish. To resolve the problem, open the Downloads window in Firefox ("Tools -> Downloads") and clear the list of downloads.