Flash Drive = USB, Yes?
Just access the it in My Computer and delete everything? s:
If its not a USB - which I dont see how it cant be - Try this..
Start > Control Panel > Administrative Tools > Computer Management. In the list choose "Disk Management", listed under the "Storage" category.
See if your Flash Drive is listed. Simply rightclick it and choose Format. If you get the same error try Delete Partition. Does the same, in the end. No partition - no files to store. You'll just have to make a new when its done.