Sounds like you may need a Linksys Nlsu2.
It's a tiny network usb device that lets you attach a usb harddrive, it then just shows up as a network share.
However as it runs linux there are modified firmwares available that allow the installation of other bits of software. I've got mt-daap running on mine, (an itunes compatible server) that allows all my machine to see the shared music from within itunes, and is therefore accessible from other itunes devices (roku soundbrigde).
Edit: you want wifi.
flash it with modified firmware and add a supported USB wifi stick, that or plug it into your router.