My USB ports seem to be working on and off on my 24" iMac. Sometimes if i plug my external hard drive in, the icon will pop up on my desktop and then immediately afterwords i get the "device removal." This is when plugging it into the keyboard. It seems to work better when plugged into the back of the computer. Then today my camera wouldn't show up on my screen at all. I tried plugging it into my keyboard and into the back. Nothing. I just used the camera last week with no problems. I tried it on my PC and it worked fine. Any suggestions!!??