I'm trying to install high sierra on a external SSD (250GB) it's formatted in mac os expanded. the installation shuts down when the computer reboots during the installation. The installation program gives me the 2 errors you see in the attached image. first it says that the disk is to small, and then it gives me a error "System/Installation/packages/OSInstall.mpkg is damaged"
I've also tried to instal from reboot (cmd+r) and then selecting repari/reinstall, on the external hdd - same errors as above. - and i've tried downloading os sierra 3 times now to make sure, it wasn't a bad file in the installationfile.
What am i doing wrong? Why is it nessesery to reboot the computer, when i'm installing on a secondary hdd.