There is so much misinformation on USB ports it hurts. Unless I missed something in one of these posts here are a few things I didn't see or were not clearly defined.
The USB ports do not run off any power supply lead directly any more than the CPU or a Ethernet port/card does. Nearly everything outside of the CPU first needs to go though a chipset -- so there isn't a direct connection from the power supply to the USB.
All the desktops I've seen for quite some time use the same general circuit for the front and rear USB ports. The ones in the front are attached to the motherboard the same as the ones in the rear it's just a different way of attaching them. The rear ports are typically soldered directly to the board, the front ones use a cable to connect to a pins, that are also soldered to the board.
What I know I didn't see here is the fact that the computer has various ports paired together in HUBS. The system may have 4 HUBS and sometimes more with 2 or more ports per HUB, (this is an internal [virtual] HUB). If too many items are using the ports on any one HUB there may be a problem. It is very difficult to determine which HUB controls which pair, (normally a pair) of ports without trial and error. They are not necessarily next to each other. On some older systems some of the ports might have been 1.1 and some 2.0 and it can get even more confusing.
I have seen any number of problems, some ill defined, when any HUB had too much draw attached to that HUB. Some devices draw more power from the HUB than others. USB powered external hard disks are among the biggest draw items you can attach. It is also possible for some internal devices to use a port on one of those HUBS.
What I'm trying to say is that it might require trial and error to find the right combination of where USB devices are plugged in. Each HUB has its own voltage total that can be used but overloading any one HUB, (or possibly all the HUBS), can be a problem. The only USB ports I have seen in the last few years that truly went bad had physical damage, but that doesn't mean they can't fail on their own. With all that said, most major manufacturers install underpowered power supplies for anything more than what the system came with in the way of add-on components so an underpowered supply could cause problems as well.
FWIW
