I had MacOS Mojave (10.14) on my 2019 15" MBP and have been running Windows 10 Pro on an external Samsung T5 Thunderbolt 3 SSD with no problems.
Today (after I made a BootCamp image AND cloned my external SSD!) I upgraded my MBP to MacOS Cataline (10.15) and the upgrade proceeded without issue.
Previously, to boot into Win10Pro, all I had to do was plug in the external Thunderbolt 3 drive before turning on the laptop, and it would boot straight into the Win10 Pro OS. Now, it does not do that and keeps booting straight into the Mac OS. I can get the MBP to boot into Win10Pro, but only if I press the option key while powering up. I NEVER had to do that before…and am mildly bummed that I have to do that now. I know it is only an additional step, but it is irritating.
Anybody encounter this? Anybody have a solution…or do I have to go back to the old way of booting into Windows by pushing the “option” button?
I await the sage wisdom that will be forthcoming from all of the knowledge and experience that embodied this forum;>
It sounds like the external drive was selected in Startup Disk and if no Windows disk was found, it booted to macOS. This probably got reset when you upgraded. Try plugging in the drive, select Windows as the startup disk, and then see if the same behavior comes back.
tim
1 Like
That did the trick. I totally forgot about the startup disk selection in the system settings.
Kudos on nailing the solution on the first go round!
Cheers
A broken clock is right twice a day 
tim