Without going into much detail, the scratch can grow to humongous sizes. In general, figure on 50 to 100 times or more the size of your largest file multiplied by the number of files you have open.
My preference is not to work with less than 200 GB of contiguous, unfragmented available drive space for scratch on a physically separate, dedicated, internal hard drive as my primary scratch disk.
Also consider that if you rely on your boot drive for primary scratch disk, Photoshop's scratch is sharing that space with the swap files of the OS.
That's one of the issues, and it can be be made much worse if you have a faulty drive. A badly fragmented drive would also be a big problem.
As for that external drive with a pathetic 8 GB of remaining available space, you should back it up, store it in a safe place, a get a new one. Any time any one of your hard drives gets to be 80% full or more, you are already in a heap of problem. With that little space left, you should make sure it's not being used even as a secondary scratch disk.
Hard drive real estate is very cheap these days.
What "scratch disk error" are you getting?
Is Photoshop telling you that you are out of disk space (because you are)?
Or is Photoshop telling you that the disk has errors? (which means you need to backup your files and repair the disk, or replace the disk)
I suspect it's just the out of space error, because you're asking for 1920x1020 inches by mistake..
Hello, welcome to the forums!
What version of Photoshop are you using. Did you run help>updates? What version does it show on help>system info?
What is the unit you selected for 1920x1080?
Edit: missed the fact that Chris catched it as well.