I'm mounting wifi in hallway ceiling; I have 110v in attic and plan on running Cat6 from structured media closet to supply internet. How do I wire for portable hard drive wifi backup? I believe it connects to the Apple wifi by USB and I don't want to put the hard drive in the attic? What wiring do I need from the wifi unit back to the closet?
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If you are just using a stand alone drive, no power adapter needed, then just run a USB cable into a closet near by or under to wireless adapter.
Not knowing your climate however choosing to put a device like an Access Point or Router in an attic sounds like a bad idea. These devices are sensitive to heat and cold and if you were living in a northern area these conditions likely won't allow your device to last as long as it should. To much heat in the summer and way to cold in the winter. Not to mention humidity control.
I would mount it all in a closet and run power and ethernet cables to the closet. This solves your hard drive placement problem as well.