connect your 404 to your PC.
enable USB Hard Drive on the 404.
Open My Computer
Right click your archos drive and choose "Properties"
In the properties menu, goto the "tools" tab.
in the Tools tab under "Error Checking, click "Check Now".
A scandisk box will pop up -- make sure the both check boxes labeled "Automatically fix File system errors" AND " Scan for and attempt recovery of bad sectors" are CHECKED.
The check and repair will take a long time, due to the fact that your archos is a USB drive, thus limited in its transfer rate to the max. rate of the USB cable. Just be patient and let it work.