Flash drives use power, relative to the size of the drive. For instance, my 1 and 2 GB drives work fine on my HHKB. My 8 and 16GB drives, on the other hand, do not.
This is a very old problem. Going back to the old school apple keyboards that had USB ports on them, they wouldn't work with anything but a mouse.
After all, that's the intent of these ports.
Oh, and any unpowered USB hub will have this same problem, as stated. It's just the nature of the spec. USB puts out 500 mA (theoretically). This is all the ENTIRE hub gets, to share. There are higher power ports, but that has to be negotiated upon plugin, but that can't be done through a hub.
What happens is the device negotiates with the hub controller, rather than the PC controller. The hub controller just says "sorry, only got this amount of power", and the device doesn't work.
Powered hubs, on the other hand, have the power to spare for these devices, as they are drawing direct power, usually from the mains. I've seen 1 hub that had 2 USB connections, one for data, one for power. It would dynamically renegotiate with the host computer in order to increase current along the power lead. However, it was just a prototype in an engineering class discussing the USB spec. Not sure if those exist commercially.
Hope that helps.