I'd strongly second what Gearomatic said about not using an internal drive that is running your system software and DAW software for audio file recording. It's fine for back-up, but during recording there's just too high a possibility that a system call will interrupt the disk operation for longer than your record data buffer can accommodate at high data rates.
I use a Macbook Pro (2.66 GHz I-7 processor w/4GB RAM) for recording with an Apogee Ensemble. The DAW is Apple Logic Pro. I've run up to 8 tracks @ 24bits/88.2 kHz and 12 tracks at 24bits/44.1 kHz, with no problems using the following configuration:
The Apogee is the only device on the firewire port and is connected through a FW 800 to FW400 cable. Screensavers, power savers, TimeMachine, and any other "background" applications that can be disabled have been disabled or removed. WiFi is disabled, Bluetooth is OFF and no internet connection is allowed during recording.
I use various external drives of between 320 GB and 1TB on the USB2 port for file storage during recording. Some drives are 5400 rpm, rated at at least 30 MB/sec and some are 7200 rpm, rated at about 50MB/sec.
No other external devices are connected to the firewire ports or the second USB port, I even disconnect my USB mouse and use only the keyboard and trackpad when recording!
No USB hubs are used.
I have a few drives which have both firewire and USB interfaces, but I always run them on the USB bus. I feel that the bus loading is lower if the data input and output paths are on separate busses ) reducing the data rate on each bus.
With an Ensemble, the firewire bus is handling the multiple tracks of data going to the processor as well as the return monitor data going back to the interface and the occasional control data. I believe that when running Logic Pro, like most DAW's all the record data must go through the processor (and DAW) before it's formatted and sent to the storage disk.
The interface does not send data directly to the drive, it first has to go to the processor and then back to the drive, so that doubles the bus traffic if they are both on the same bus.
Rather than send all 12 tracks back on the firewire bus to the storage device, I'd rather send it to the USB bus mounted drive. Processor loading is about the same having to format for either firewire or USB, but the firewire bus traffic is much lower.