Hi John
I only have the drive connected and I have tried a HSF+ drive and an exFAT drive alternatively. My 2.5″ USB drive works okay though with FCP X.
I actually have two FW buses because I have one built in and one with two more ports on a PC Express card (the last 17″ MBP to have a PCExpressCard slot). Even so Drives won’t display in FCP X.