Bootcamp! Starts under Parallels, not as standalone startup disk

Hello
With Parallels Desktop 15 for Mac, Pro Edition 15.1.2, running on an iMac host having Catalina 10.15.3, the Windows 10 Pro guest OS is installed on an external SSD. Windows boots and runs under Parallels, but not standalone.
The Windows installation has a history of having booted standalone i.e. recognisable as a startup disk. I was even able to run the Windows OS on other hardware. I'm not sure if I lost the capability of booting standalone when I moved from a 2012 MacBook Pro to the current iMac (with Fusion Drive), or soon after when I updated to Catalina, or even when I used diskutil to alter the 2nd of 2 partitions on the SSD. The guest OS was not always Windows 10 - it was 7 Ultimate, and the loss of standalone startup functionality pre-dated my updating to Windows 10 (I hoped the update would fix it).
This may be more of a question for the Mac community, but having searched it, I find it is fragmented so I didn't know which way to turn - except my fellow Parallels users: essentially, how do I keep the Bootcamp partition on the SSD but restore the capability of using it as the startup disk. There are some very helpful prior posts but they don't seem to offer the right solution because they reference disk volumes that aren't the same of the ones I see with diskutil. This could be confused by the Fusion Drive. I know I need to get over that hurdle, but seek a helping hand, please.
All the best
Matt