Otdfbt

I have two exact copies of Windows 10, one on drive C: and one on drive D:. These are two internal drives in the same laptop.

How can I know which Windows is running?

How can I select the one to launch during boot?

The drive letters like C: are only assigned during Runtime of a Windows installation, so they will usually be both C: for both your installations.

Open an elevated cmd.exe or Powershell.exe (as Administrator), then type:

diskpart

after a moment, type:

list volume

You get something like this:

 Volume ### Ltr Label Fs Type Size Status Info
 ---------- --- ----------- ----- ---------- ------- --------- --------
 Volume 1 WinBOOT FAT32 Partition 350 MB Healthy System
 Volume 2 D Win1 NTFS Partition 31 GB Healthy
 Volume 3 C Win2 NTFS Partition 33 GB Healthy Boot
 Volume 4 S Data NTFS Partition 380 GB Healthy

In this case you can see that the Windows on Volume 3 is currently running, there is another one on Volume 2, Boot in the Info column on the far right indicates that this is the current Windows Drive.

you can see the same by open Disk Management:

 diskmgmt.msc

and look for Boot in the Status column of volumes.

When setting up a dual boot Windows you should use descriptive names for each boot entry.

There are few ways, and I'll mention quickest ways that comes into my mind.

1. 
   

   Open Run dialog (Winkey+R) and type below and enter, it'll open your
   current windows installation directory

   
   
   

   %windir%

   
   


2. Open Task manager and select a system process(something like
   svchost.exe or winlogon.exe ) in details/processes tab. right click
   on that and you can see Open File Location, which will also open
   your windows directory.

Some other ways

• Open start menu, type "system info" and open System Information. You'll see the Windows Directory under System Summary
  


• Open start menu, type "disk management" or press Win+R > diskmgmt.msc > Enter. That'll list all the drives in your PC. The system drive will be listed with the Boot flag, most likely along with "Crash Dump" and "Page File"


• 
  

  Simply press Win+R and run cmd. The path to cmd.exe is often shown on the title bar by default. On some systems it simply shows "Command Prompt" but in that case you can look at the prompt string which often points to your user folder in the system drive

  
  
  

  

  
  
  

  This is quick but not reliable either, so while you're at the commmand prompt just run the commands in other answers or you can run echo %systemroot% or set sys

  
  
  

  C:>set sys
  SystemDrive=C:
  SystemRoot=C:WINDOWS
  

  
  


• 
  

  Press Win+Pause or right click My Computer > Properties > System Protection. Your system drive will be listed with the word "System"

  
  
  

  

  
  

That said, nowadays Windows often shows its system drive always as C: regardless of its system volume position in a drive, so a drive label or device name is more useful than a drive letter

Open CMD an Type in wmic OS GET SystemDrive /VALUE

This Will Return System Drive Letter.