Disk not readable message when using
From your question it is not clear what you put into those USB ports. Memory sticks appear to the system as a disk, so the message you quoted might be caused by a memory stick that had been formatted in a file system your Mac does not know (possibly from a Linux computer?).
If you want to widen your possibilities, you might consider installing OSXFUSE. But I would appreciate if you first clarify what you mean by "use either of the two usb ports".