Which button is disabled? I never meet this before.
Are all the format supported by the router?

Could be a non powered hub struggling to power too many connections at once,

Although I can't say for sure but I do seem to remember seeing somewhere that there is a maximum of devices that the usb can handle and it might have been only two, can't remember where but I think it was in one of the user manual downloads so check yours to see if there is a limit or not, hth.