I've got this stack of old Minis that I'm trying to rehabilitate by putting cheap SSDs in them. (I've got some time on my hands, and it beats them becoming landfill.)
I've done this several times before, and mostly it has been going according to plan, but I've got this Mid-2011 i5 that I just can't figure out. I can boot it off an external disk that is running 10.13.6 (the latest OS that this hardware can run) but nothing I try -- booting the recovery partition from the external drive, booting internet recovery from BIOS, or running the "Install macOS High Sierra" app when booted off of the external drive -- leaves me with a bootable system on the new, internal SSD.
It never updates the new disk's partition table to have an "Apple_Boot Recovery HD" partition on it, which is probably not a good sign, but beyond that, it's never able to boot the second phase of the installer. "/macOS Install Data/" exists on the new drive, but it panics with "error loading kernel cache".
If I rsync the contents of the external disk onto the new internal SSD, I can boot off the SSD -- but only while the external disk is plugged in.