"Disk on key" what does that mean?
Do you refer to get an ISO file onto an USB stick?
If you're having 3 physical harddisks in your computer, then yes.
Otherwise, i dont know if it *maybe* looks for an usb-external-hdd?
First off, open a partition tool, dont change anything, but have a look for the partitions/disk names/descriptions.
Look for /dev/sdaX /dev/sdbX /dev/sdcX
Where X can by any number, and refer to the partition 'id' of that drive or media.
Out of personal prefernces i'd suggest you the following:
dd if=/path/to/file.iso dd=/path/to/usb/stick
dd if=fedora15-live-lxde-x86_64.iso of=/dev/sdb1
Where /dev/sdb1 is the first partition (with bootflag) of my usb stick.
Boot from USB and try reinstall.