From iCloud Storage to External Hard drive

My iMac keeps shutting down and I need to get it fixed. I went to the Genius Bar and they recommend thatI do a backup before it is worked on. I have an external hard drive however, the computer shuts down before I can get my computer backed up. Is it possible to upload everything to iCloud drive or whatever and then transfer it from "iCloud" to my external hard drive? If so, how do I do this?

Thank your for your help and advice!