How to safely switch off from internet recovery mode

I have got myself into trouble attempting to reinstall El Capitan onto an erased Macintosh HD.

Now my Mac Book switched on automatically to internet recovery mode and unfortunately my external backup Seagate drive is plugged in to the USB port.

I seemingly cannot control my machine while in this Internet Recovery mode. However, I am afraid of turning off my machine at the power button with my backup drive plugged in (the drive which now contains my data since I followed the step of erasing the Macintosh HD. I did that in order to restore my hard drive with El Capitan via Time Machine using Disk Utility from recovery mode)

Can someone confirm or not if there is a risk to my backup drive by switching off by the main power button?

Is there another way of shutting down in this situation with the least possible risk of damage to a plugged in external hard drive?

More background, if it helps:

I originally wanted to upgrade to OS Sierra in order to use an app which wouldn't work on El Capitan.

However, once installed with Sierra my machine was much slower, sometimes freezing up completely.

I did not know that reinstalling an earlier OS (in my case El Capitan) was not just a simple matter of downloading and mounting the disk image. When I tried this, I discovered that I would need to erase my disk (at least, I couldn't find another way described online).

Because I had a full Time Machine backup on my external Seagate drive, I thought it was risk free to proceed.

So I erased the disk using DiskUtility in recovery mode.

I did not expect that, after trying to boot up in recovery mode again after the disk erase, it would automatically run Internet Recovery Mode, which effectively renders my machine unusable, with no way to access Disk Utility or eject my external hard drive ( which may or may not be mounted, I have no way of knowing).

I feel now it was foolish to erase my main disk, but then in doing so I was following advice from another apple community page. The necessity arose because when I tried to use Time Machine to restore my original system before Sierra, I had got the following message in Disk Utility (recovery mode)

This system can't be restored onto this disk because HFS systems can't be restored to space sharing APFS volumes. Reinstall macOS on this disk and then use Migration Assistant to transfer data from your backup instead.