I've got three external HDDs, all My Books, 500 GB, 750 GB, and 1 TB. If I hook one or two of them up to a computer, everything seems fine. If I hook all three of them to a computer (I've tried this on two different computers, one running Windows XP, one running Windows Vista), the bootup time goes up by more than an order of magnitude. It can suddenly take about 10-20 minutes to boot the machine, whereas before it might take a minute.
All three drives work fine on their own. I'm not using any kind of hub; all three are plugged directly into the machine.
Is this associated with some kind of inherent limit in USB? Is this bad hardware design in the CPU box? Is this a My Book problem?