i tried to move a large file to iCloud drive and ended up with a full boot disk. Help!

I have a relatively small (by current standards) SSD on my old iMac, so I have a number of external drives that house things like iTunes and Photos libraries (and backups for both). I have several Photos libraries.

Anyway, yesterday I had the bright idea of copying (uploading) one of the Photo libraries to the iCloud Drive. Since this is a fairly large drive, I knew it would take some time to upload (days, perhaps). Unfortunately, when I returned to my computer several hours later, I got a warning that my startup drive is nearly full (very, very nearly full). I assume that the attempted operation loaded data into the startup drive therefore filling it up. One of the problems is that I can't find where that data has gone so that I can delete it and free up the space on the drive. That's the issue I need to address immediately. As it is, I can't do much of anything on that computer.

Obviously the operation I attempted wasn't the right thing to do. I have plenty of space in my iCloud drive. What is the best way to move libraries like that up there?

Thanks,