Using external drives for extra disk space?

I have a Macbook Air with 256 GB of disk space. It seemed like enough at the time. I currently use Sierra. I am waiting for a mouse driver update before updating to High Sierra. I know that will open a few more options. I am down to 29.83 GB free. I have been triaging, deleting, and de-duplicating apps and docs. It would take more and more work for less and less free space.

One suggestion is to move less-used files to a cloud drive or to an external drive. How can I do this, so that the files will still be available if and when I need them? And will be backed up on the other time machine drive, in case anything happens to one drive?

I only have two usb ports. I think some newer Macbooks have only 1 usb port, which isn't enough. I use the left port for my time machine drive, or sometimes for my cd/dvd drive. I use the right port, and a usb hub, for an external keyboard and mouse.

Most external drives have flashing lights to indicate when they are in use. I get migraines from these lights. I can keep the time machine drive behind the computer, where I don't get migraines from it, but apparently this is too close to the Wifi antenna, and can cause problems with the drive.

Is there any safe place to put external drives, where they won't cause migraines or seizures, and won't go bad?