I have a mid 2012 15" unibody MacBook Pro with one drive being a 500 GB SSD with Mojave onboard and a second 1 TB mechanical hard drive running High Sierra for use with Win 7. I noticed that after setting up Win 7 on the OSX High Sierra and it being a drive on a hard drive caddy, that Win 7 when restarted, would not boot giving an error stating that I had to have the boot disk for the os loaded. I solved that error by putting the drive with High Sierra as the main drive and the SSD with Mojave on the afore mentioned drive caddy as the secondary drive. Win 7 finally booted and I was able to also get into Mojave successfully. I just wondered if anyone else had experience this problem. I am also contemplating partitioning the Mojave drive and put Win 10 onto it. Your probably wondering why? Well, some of the old Windows games (circa Win 98, Win XP) will not play at all on Win 10 even with enabling mode used, and I like my old games even though I have some newer games that can run on the Mac.