the one caveat to using externals i can think of is it would be better if using multiple external drives to have one usb then a firewire if you have to have two

I did try using 1 usb for external runtimes then another usb for textures. after about a month I began finding there had been write errors (mostly my own fault for sticking with the fat 32 they were preinstalled with instead of ntfs - but some were delayed write errrors where too much data went on the usb for it to handle)