It's kind of slow. I don't know if it would be too slow relative to how you're using samples or not.
I used to use a Firewire drive for samples on a laptop; I now have the same drive hooked to a desktop via eSATA (which makes it almost as fast as an internal drive) and the speed increase relative to Firewire is very noticeable. Load times for large projects are much shorter. The playback streaming performance is, I'm sure, also much better with internal/eSATA, but on the laptop I was pushing the limits of RAM and CPU already, so I'm not really sure how much of a playback bottleneck the Firewire connection was.
Another possibility is to buy a larger replacement internal drive for the laptop, then use a USB enclosure to transfer things from the old drive to the new drive. (EDIT: n/m, forgot that you were pulling the second drive from an old system, not buying it.)
EDIT: Or you could use the USB drive for games and data and put Kontakt on the system drive. I don't know how using a USB drive for games would impact game performance, but it probably wouldn't be as adversely as using USB for samples would impact sample streaming performance.