External Hard Drive Dilemma: USB2 or share the FW400 bus with Konnekt48 and FW drive?

[CZ101]

I'm about to invest in a new external hard drive for scratch/playback duties (DP 5.13 and Logic 8.02) and I had planned on just ordering a USB2 drive because it would be cheaper than a dual port USB2/FW400 drive - and because I wanted to reserve the FW400 bus on my iMac C2D 2.13 exclusively for the SK48.

However, in reading a lot of people's warnings about USB2 external hard drives not having enough bandwidth for audio, etc, it has me wondering - should I get a dual port USB2/FW400 drive and ... should I expect to have to use the iMac's 1 firewire bus (2 ports) for both the SK48 and the external drive?

Is anyone successfully doing this on an iMac (Late 2006) 10.5.6 - using a FW external drive sharing the FW bus with an SK48? Or, is anyone successfully using an external USB2 drive with the SK48 as the only firewire device?

I've read that USB2 drives will drain more if not considerably more CPU resources than a FW external drive, so this is another consideration.

thanks for your help,
Paul