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#2370
Connect both disks. boot up into whichever system is booting. Find out which is the drive letter for the system volume with Diskmgmt.msc (Admin). It should have a system hidden folder called BOOT when viewed in file explorer (if you have the folder options set to view system and hidden files and folders. for demonstration purposes, lets call this drive s:
Find out what the drive letters of the current booted WIndows (c:) and the one that isn't booting (e:).
For the one that isn't booting open an admin command
+ x a
then type
bcdboot e:\windows /s s: <enter>
This should add an entry to the BCD (Boot Configuration Database) boot menu on the system drive s: for the windows system on drive e:
change the drive letters to suit your drive letters