Boot Camp Assistant - External storage device Attached

[drmeatball]

Small problem, maybe someone has advice?

I run my 2018 Mac mini entirely on an external SSD. Works perfectly. Internal SSD is wiped. I'd like to use the internal SSD just for Boot Camp. However, when I run the bootcamp assistant I get this:

Kind of a catch 22.

I'm guessing this is the answer.. do I need to reinstall OS X on the internal drive just to be able to run Boot Camp?

 

[DeltaMac]

You don't need the Boot Camp Assistant (BCA) to install Windows on your internal drive.
Just boot to the Windows 10 installer, and choose to format the internal drive. The Windows installer will take care of making that internal drive Windows-bootable. (And, you SHOULD disconnect your external macOS boot drive while installing Windows, so that you (or the Windows installer) don't make any accidental choices that format your macOS volume. Better to leave that disconnected during the Win install.
You CAN use the BCA to make your USB bootable Windows 10 installer, and also to download the Boot Camp support software. But, BCA won't let you make a Windows partition when booted to an external drive (because it expects to use the same drive that you are booted from, and Windows, by default, can't install on an external drive.
(My statement seems wrong, after I tested BCA on different external boots. Never see that "external storage" message. But, it always detects my internal boot camp partition, so maybe that's the difference?)

 

[DeltaMac]

I tried booting to external drive, then tried Boot Camp. I got no such message, even after connected another storage drive with multiple volume. In each case, the BCA would detect my internal drive, which has a Boot Camp partition, offering to remove that partition. But, no message to disconnect external devices.
Are you using a TB boot drive?