I've learned a lot. Thank you, Damian, for some good pointers.
I don't think disk migration was the problem, however. Here's what I found out:
I was in the process of turning off replication for each folder, one at a time. It was taking a LOOONG time, about 15 minutes PER FOLDER, because the CPU was slammed.
While I was waiting for console to finish changes to one folder, I idly RDP'ed into WHS again, opened Task Manager, and this time checked "show processes from all users". Suddenly I had a whole page full of things running, and on doing a sort by CPU usage, found TwonkyMedia at the top, using 99% of CPU time ! I hope I didn't break anything in WHS, but I forced the TwonkyMedia Tree process to stop inside Task Manager, and suddenly, the 490 came back to more normal life. I don't use Twonky, and have no use for it, so I may just disable it permanently once I'm sure nothing else is amiss.
Next, I'm going to restore folder duplication and watch things for awhile.
And now the $64 question is, why was TwonkyMedia running amok, and if I don't kill it permamently, what should I do to diagnose and prevent this from happening again?
Thanks all, especially Damian, for encouragement and advice.