I had just this problem with my 3520A in a very generic outboard USB/Firewire case.
A few minutes ago, (while turned off), I unplugged the Maxtor OneTouch hard drive M: that uses the front panel firewire port and changed the NEC, drive N:, to that connection; after reboot I flashed the May 1 '05 L&D executable from that .nl site. Flashing worked!
After putting the plugs back the way they'd been, all seems fine. But, the 3520A is now drive I: and the Maxtor is drive H: