What OS is running on the machine, and is your flash drive just a USB device?
One thing that you may need to consider with a clean format is that the machine may be missing USB driver information to maket he flashdrive communicate appropriately. For my Iomega flash drive there have been times that I've had to download the drivers for it before the computer would recognize it and allow me access. Or you just pooched the USB driver for that particular drive because you had it plugged in when the OS installed so now it thinks it's a hard drive instead of removable storage
