Exactly what I want to do (although I may use a regular hard drive for Windows). I have posted a question on this, which as not been answered. as yet. Your experience does not fill me with optimism, sadly.
I have seen the “inaccessible” message before in virtual set-ups, using the Boot Camp partition, and I had tried every trick in the book, eventually only coming right through a Parallels upgrade. Quite what was going on, I never established. A while back, I had exactly the same problem with VMWare Fusion, and that’s when I switched to Parallels. The Boot Camp partition always booted properly on its own. So, although we are looking at a different problem, this message is pointing to a similar glitch, I would expect.
The issue is most likely drivers since Windows needs the correct driver during boot up for the TB3 enclosure. Since apple doesn’t provide it, it would fall on the vendor to provide it, so it might be worthwhile to check their website. I wouldn’t be surprised if it didn’t exist since there probably isn’t a lot of folks external booting Windows with it. I could be wrong though
The drive shows up when I load windows thru the internal boot camp installation.
Also, the drive controller is common (Intel’s JHL6340).
I think there’s something else that is causing this… such as TB3 drive mount timing with the boot load sequencing.
I think someone mentioned that it works if the drive is disconnected and reconnected right after the windows boot sequence?
I got all of the drivers working except my external thunderbolt 3 devices, so this is the issue. The only item in device manager with an issue is “unknown device”. I have tried removing bootcamp drivers and reinstalling and tried Sysprep