I have the same problem. Two different EZQuest Quad interface drives suddenly won't mount on my 2010 iMac (Core Duo) via the firewire 800 or 400 ports. If I use a USB cable, both drives mount as normal, but I need the Firewire 800 ports to stream audio samples for Logic.
Both of these drives were working fine for over a year until I upgraded recently to 10.6.8 on the iMac and then tried to use one of the drives yesterday with a new 2012 MacBook Pro (10.7.4). The drive mounted once on the MacBook Pro, then disappeared and now will not mount on either the MacBook Pro or my iMac.
I tried resetting the PRAM and SMC on the iMac, but the drives still won't mount via firewire. The fact that both EZQuest drives still mount via USB indicates this is a firewire problem particular to Mac OS 10.6.8 and 10.7 rather than an external drive problem. It also seems that many people are experiencing this regardless of the external drive brand. Does anybody know how to fix this?