I attempted to preclear three of these drives simultaneously on my box, but ended up with two processes hanging and the third continuing to run. I think it may be a function of not enough RAM in my box (only 512mb until I put another gig in it next week) since the web GUI has also hung and I can't launch any new processes on the box.
I'm at 33 hours for the first drive thus far, with my read and write speeds varying from 50MB/s to 114MB/s. I'm going to reboot after preclear completes on the first drive and then try to run the next two drives concurrently.