My Mac Mini connects unreliably to wifi network. Slow to no connection. When a usb drive is attached connection degrades to nil. When drive is removed the connection is flakey: could be very fast and then drops to dial-up speeds. Other Macs on the same network connect at close to nominal fiber speed. Apple told me to upgrade OS and see. I tried downloading OS installer, installing on external drive, and booting from same. Could not connect, so I conclude software is not the problem and tat maybe USB is starving wifi of power or wifi unit is bad and that is why wifi connection fails. If USB is fried does that mean I need a new system board? If wifi is bad, that subsystem can be replaced. Correct? Fortunately I have Applecare. I'm hopeful they can fix my mini since it has upgraded RAM and drive. Any insights appreciated.