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If you mean it won't show the external drive on the desktop. Go to preferences for finder and make sure the show external drives check box is checked.

In either case the drive should appear under "devices" on your finder window sidebar.

If not check if "disk utility" detects your connected external drive on its side bar.

In the case of using a USB hub to connect your external, try subtracting the hub & connecting the drive directly the computer and see if that changes anything.

If you are indeed already using one of the USB sockets of your Mac, try using the other USB socket and see if that brings up the external on your Mac.

Hope any of this helps.
 
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