Not enough space when installing os x

I am trying to install OS X High Sierra on an external USB SSD-drive with 256 GB capacity from my 2014 Mac Mini in order to boot from that drive in the future.

The drive is formatted to OS X Journaled, and there are no files on it. I try to install OS X on it, and everything seems to work until I reboot - then I get the message "Not enough space on hard drive". But when I check the disk, there's plenty of space on it - more than 200 Gigs.

I did this kind of installation on another computer with the same harddrive, and it just worked right away. Does anybody know what the problem might be?