External hard drive knocks out wifi Internet
If one of your external drives is connected using USB 3, then this is most likely the culprit. If the shielding has degraded on the USB 3 cable then it can interfere with your wi-fi. If you external has any other type of connectors, i.e., USB 2 or FW800, switching to that will solve your problem.

This excerpt from this PCMag article explains why this can happen...


  USB 3.0 has a 5Gbps signaling rate. The USB 3.0 specification requires
  USB 3.0 data to be scrambled and it requires spread-spectrum. The
  noise from USB 3.0 data spectrum can be high (in the 2.4-2.5GHz
  range). This noise can radiate from the USB 3.0 connector on a device
  (such as a PC or Router), the USB 3.0 connector on the peripheral
  device or the USB 3.0 [port]. If the antenna of a wireless device
  operating in this band is placed close to any of the USB 3.0 radiation
  channels, it can pick up the broadband noise. The broadband noise
  emitted from a USB 3.0 device can affect the SNR (signal-to-noise
  ratio) and limit the sensitivity of any wireless receiver whose
  antenna is physically located close to the USB 3.0 device. This may
  result in a drop in throughput on the wireless link.