you should be fine with a battery pack with a 5v regulator (ie- 7805 or similar) hooked up to the power pins of the power-only part of the usb cable.
the only problem I see is that when a computer powers down, goes into standby, or hibernates, it turns off the USB ports. at best, you'd be wasting battery power running the hard drive with the computer off, at worst you may damage the USB port by holding the power at 5 volts when it is supposed to be off (you'd have to find out if the USB port power is floated, or pulled to ground, when it turns off) not to mention, you'd probably be powering other USB ports with your battery pack also depending on how you wired things up, so when the computer turns off it may drain your battery very quickly.
you could rig it up so that the battery pack is only turned on when 5v is detected on the primary (data) usb connector.