Tim:
Sounds like you're now relying on having a hub that will supply more current than it should. Cross your fingers that hub builders don't start adhering to the USB specifications... ;-\
Alan Cox:
Or have a PC which is underpowered for some reason - that happens too.
Or one that has many USB ports on the same internal hub, and between all the peripherals you have attached, you've over-reached the limit.
I don't like USB-powered disc drives, for those reasons. They need just all the power that's possible for a port to supply, and anything else taking power pushes it over the limit. Some drives come with a USB Y-lead, to take power from two ports, but that's not going to help in some cases, and it bothers me that I don't know whether they'd dumbly wired the power lines directly together, or have safely joined them (through something like diodes).