I added to my posts re: Windows To Go, also talking a bit about just moving a Windows 10 installation to a USB drive, along with copying 10 from a USB drive to a VM for easier updating, then copying it back to the USB drive. [BTW, the same stuff would work for a VHD -- since Windows 7 you can boot to Windows on a VHD the same as if it's on a partition.] On Saturday there was a partitioning app on GOTD... a couple of the comments for that GOTD, plus the stuff I was talking about in the WTG post, led me to believe I should say something briefly about disk & partition IDs. [*Briefly* because this can get both complicated & boring real quick, & hopefully it's enough that you know it exists, so you know more where to look if things go wrong.]
Drives & drive partitions have unique IDs, because drive letters can always vary & be changed. Linux folks & those working a lot with VMs are more prone to know about & work with these IDs than Windows folks, but with Windows they're equally important.
If you have 2 disks &/or partitions with the same ID, one will be disabled -- you can't have the same ID twice at the same time. This usually becomes an issue when you're cloning or copying a disk or partition, so the software you use may or may not change the ID on the new copy or clone, and whether it does change it or not may depend on the circumstances, or not, e.g. a partitioning app may change the ID every time you use it, or never. It can also lead partitioning &/or backup &/or cloning software to alter the boot files &/or the IDs, & that can sometimes be controlled in the app's option settings, e.g. Paragon B&R 15.
Since Vista, Windows has used BCD for booting, & it not only goes by those disk & partition IDs, but can create or change them. Disks that are formatted MBR use one disk ID, while GPT disks use a larger GUID. Windows boot files have a registry hive [you can open BCD in Regedit] & Windows of course has registry hives, & the disk/partition ID is stored in both, along with being stored in the drive or partition tables. [And yes, this IS the brief version.]
If you do something like copy or clone a disk or partition, you won't be able to open or access both at the same time unless you research the arcane measures you have to take to change the ID on one of them. Most of what you'll read simply says Don't Do This, meaning don't expect to access 2 copies or clones of the same disk/partition at the same time.
If you wind up not being able to boot into Windows, or a copy of Windows if/when you dual boot, run EasyBCD [free & works without install] & redo the boot menu. Unfortunately you have to boot to a copy of Windows to run EasyBCD -- that can mean connecting the drive to a working Windows system, or having Windows on a bootable USB drive etc. You can also use the [more confusing] Windows command line apps, BCDEdit & BCDBoot, but like EasyBCD, you'll need to boot to a copy of Windows or WinPE to use them. And yes, this stuff is another reason to create disk image backups.