I used to have an XM1 (the UK version of the GL2's predecessor) and I often found that FCP would not properly recognise the Cam with an Firewire Drive connected (even via a different FW port -FW400 Cam and FW800 Hard drive).
BTW -MacBook Pro 2.33GHz, 3GB Ram, FCP5.1 OSX 10.4.something (at the time I had this problem)
I seem to remember reading that the XM/GL/1/2 Cams had issues with FCP (or vice versa) but I can't seem to find where I read it. (Got rid of the XM1 a while back)
What I ended up having to do most of the time was rebooting without the Hdrive and capturing to my system drive then exiting FCP, disconnecting the Cam, connecting the Hdrive, moving the footage to an appropriate project folder in my Ext Hdrive capture scratch and then importing it into FCP.
A bit of a pain -particularly if you're capturing more footage than you can fit on your system drive at once.
Never found any other consistent solution to this problem -hope this helps.