An annoying aspect of the current Mac hardware experience, with FireWire on the way out, is that external storage no longer “just works.” Nearly every USB hard drive I’ve used—across a variety of Macs, enclosures, hubs, UPSes, and buildings—has either exhibited power problems or random disconnects. Moreover, I’m sure that these power issues are widespread, but they are rarely discussed in product reviews and spec sheets.
Particularly problematic are the bus-powered 2.5-inch drives. The USB 2.0 Toshiba Canvio is the best that I’ve used. As far as I know, it operates correctly with the specified amount of USB current. But try connecting more than a couple to a USB hub, even the NewerTech one that boasts of delivering “all the power needed for all seven ports,” and watch it disconnect or fail to mount. The other hubs that I’ve tried, even from reputable companies, are worse.
The USB 3.0 Toshiba Canvio is even more problematic. It draws more power than is allowed by the USB spec, so that it doesn’t work at all with newer MacBook Airs and MacBook Pros that supply only the specified power to the USB ports, although it can work via a powered hub.