Make sure you use an external power source for the enclosue, USB3 devices often require more power than USB2.
Thanks, JustSomeJoe. I know that USB 3 devices require more power, and I also understand that USB 3 ports also supply more power.
I solved the problem (I think). I first formatted the disk and answered the question, "Would you like to use this disk as a Time Machine backup disk?" in the affirmative. Then it didn't work. When I tried reformatting the disk, I said no, and now the drive works fine in the USB 3 enclosure. I have no idea why that would make a difference. I'm now using the disk as a Time Machine disk, and it has worked several times, and without a powered hub. Perhaps it was just coincidence, but it was the 3rd time I formatted the disk, and the only time it worked.