A USB 2.0 Cable should have 4 cables.
They are the Vcc, Data+, Data- and Ground.

Will only disconnecting the Vcc **OR** Ground Wires damage the Connected Device or the Data Wires?

I currently have a USB SD Card reader that I like to Disconnect and Connect. The reader is mounted on the table, so to disconnect the reader, I would plug off the USB Cable connecting the reader and the computer.

To solve this problem, I would like to place a switch in the power lines, but I wonder if this will break the device since the Data cables are still connected but the power lines are disconnected.

Any answer will be Appreciated. Thanks in Advanced