I've had problems with one long (4 meter) USB Mini-B to USB Type-A cable not being able to boot a 2.5'' external hard disc due to not supplying enough current. On top of that, the cable used a Type-A to Mini-B adapter for the Mini-B part, which probably made things worse.
Three different shorter cables I got around made the hard disk work without extra current, so it was definitively the cable's fault. However, if I plugged the hard disk to the power, and used the long cable just for data it worked.
Here is some related information on powering through USB cables:
I have not any long cables that don't have an intermediary Type-A to Mini-B adapter to try them out.
My question is: is there a way to guess if a cable will provide enough power for charge/disk drive power? Is it related to the length of the cable, to the build quality of the cable, or the fact that uses intermediary adapters?
Thanks for the answers guys. The computer is a Macbook Pro 13' (2010), so the USB connector is directly soldered to the motherboard (no front panel USB).
Up so far the most convincing explanations seem the ones from user12889 (the USB device being over the spec and causing problems because of tjat) and Tog (the adapter in the cable causing the power loss), although everybody else provided useful information too.
I ordered a 2 meter official USB 2.0 SPec, USB-A to USB-Mini-B cable, without any adaptaders whatsoever, and will update with the results when they arrives.