I'm trying to downgrade from Mojave back to High Sierra, which is now seemingly impossible. I used my iMac to download High Sierra from the Mac App Store and created a bootable external hard drive to try and reinstall High Sierra. I tried booting from the external HD to begin the install and I get a circle with a cross in it (almost like a stop sign). I've tried the recovery tool and the MacBook Pro ends up just looping thru the recovery process but never installs High Sierra. The only thing I can manage to do is reinstall Mojave. Would it be possible for me to create a time machine backup of my iMac (which is running High Sierra) and launch the recovery tool on my MacBook Pro and restore from the time machine backup? Any suggestions?