I have two external hard drives attached to my 2012 Mac Mini running El Capitan. All has been OK for around 2 months. Now, when one drive powers up, it forces my Sky WiFi Internet connection to drop, which you can't get back until the drive is powered down again, after which the `internet connects straight away. Any ideas (anyone)?
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.
I've had the same problem on my Macbook Pro Retina 2017 when connecting an external hard drive through a dongle.
That Mac has two thunderbolt ports, one very close to the other. I've managed to solve the issue by unplugging my dongle from the port closest to display and plugging it into a port that's a bit farther from display.
I've read here that the reason for this problem may be that HDD is stealing power from WiFi when hardware is too close to each other, or something like that. Anyway, plugging it into another port solved the issue for me.
UPDATE: It seems like the solution was a one-time. If that doesn't work, try plugging your laptop into power. Maybe even moving an extern disk as far away as possible from mac. Plugging into power solved it for me this time.