Thanks, Tony. If I come across this issue on another Mac I'll remember this.

The drives are certainly inexpensive but they work at expected speeds on every PC I've tried them on.

Another behavior I forgot to mention was that much of the time, it appeared that the transfer would move some data, then pause, move data, then pause, etc. Nothing indicated this, just that the amount of data moved would show this behavior. Could be the Spotlight issue I guess...