I thought I’d post a quick summary of the journey to getting Windows 10 to boot on my iMac using an external SSD drive.
I first tried this approach using VirtualBox. There are several posts that document similar approaches. This quickly - for anything that involves installing Windows - produced an external SSD that would boot on my 2013 MBP. But in spite of trying several variations of this process I was never able to get it to boot up on my 2013 iMac (Fusion drive model). I’ve found Fusion drives to be very fussy so I’m not totally surprised but had been hopeful because I was using an external disk and so the fusion drive shouldn’t really have been a factor. Last year I created a Boot Camp partition on the Fusion drive and that required some serious shell level disk hacking. Since getting the external SSD to work for Windows would enable me to extend the life of the iMac quite a bit I was motivated to find a solution so I kept digging.
I eventually tumbled on a completely different approach which is based on a Microsoft technology called Windows To Go. Even with this it took me several tries to get it to work and even then there were some oddities. I only had a Windows Home license and the approach documented here wouldn’t work with that version (at least not for me, Windows To Go is stated as not supporting the Home edition but I had given it a try anyway). Having already fussed with this for more than half a day I coughed up the $100 to upgrade to a Pro license. Once that was done I started from scratch once again and ended up with a Windows install that successfully booted my iMac. It performs pretty well i.e. much much faster than the Fusion boot camp partition. One of the resulting oddities is I can’t use the Boot Camp tool in Windows to boot into Mac OS X. I have to do a restart, hold the Option key, then pick the macOS boot disk. The second oddity is similar: the SSD no longer shows up as a bootable option in System Preferences on the iMac. It’s necessary to hold do the Option key when restarting and then pick a disk which is named “EFI Boot”. If you’re thinking of using this approach be sure to check out the first comment on the 9to5mac blog which points out an upgrade limitation which might eventually matter.
I’m hoping this will also have the advantage of allowing me to boot this Windows SSD on either my iMac or MBP should the need arise. But knowing Windows I’ll hope to avoid trying this out.