Imac won't detect external harddrives

i have never had this problem before. my iMac used to pick up on my WD passport and other HDDs. but for the past two days it won't detect anything. not even a thumb drive. help!?

Is it just storage devices, or all USB peripherals?
Try shutting down and disconnecting all of your USB peripherals, except for your standard keyboard and mouse. Disconnect any USB hubs. Restart the Mac and only connect one USB storage device directly to your iMac. Does that one device work? Try it on all the ports, if one does not work.
You can also try starting up in Safe Mode
http://support.apple.com/kb/HT1564
http://support.apple.com/kb/HT1455
in case there is some non-Apple system extension that is interfering with USB storage devices.

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