I have a HDD attached to my AirPort Express through a 7-port USB hub. How come it doesn't show in my iMac?
That's because the AirPort Express does NOT support external USB HDDs regardless if they are attached directly to the Express' USB port or via a USB hub.
To add to what Tesserax said -
The USB port on the Express only supports a USB printer. You will need to connect the hub to your iMac.
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