Run two read-only checks in Windows

Start from the Windows installation you want to keep. Open Windows Terminal with a PowerShell tab. If the storage commands report an access error, reopen Terminal as administrator. Leave both drives connected while you check.

$letter = $env:SystemDrive.TrimEnd(':')
Get-Partition -DriveLetter $letter |
    Format-Table DiskNumber, PartitionNumber, DriveLetter

Get-Disk | Sort-Object Number |
    Format-Table Number, PartitionStyle, IsBoot, IsSystem

The first command maps the running Windows drive letter to its disk number. In the second output, find the row with IsSystem = True. Compare Number in that row with DiskNumber in the first output. Microsoft documents the disk flags and how to query a partition by drive letter.

Windows uses boot for the partition containing the running operating system and system for the startup partition. On a typical UEFI installation, the startup files live on the EFI System Partition, or ESP. The ESP usually has no drive letter in Windows. A successful sign-in with both drives attached can therefore coexist with a startup dependency on the old drive. Microsoft's UEFI partition guide

Decide what the disk numbers mean

  1. Windows and the system partition are on different disks

    For example, Windows is on Disk 1 and IsSystem = True appears on Disk 0.

    If Disk 0 is the old SSD, keep it connected. The running installation still has a startup dependency to resolve before you remove or erase it.

  2. Both roles point to the drive you intend to keep

    The Windows disk number matches the row with IsSystem = True.

    Both startup roles are on that disk in the current session. Back up the old drive and confirm a standalone boot before you erase it. The old drive may still hold files or recovery tools.

  3. The results are missing, unclear or contradictory

    Check the partition-level result before acting.

    Confirm you ran the commands from the installed Windows session. If you still cannot identify the drives, leave them connected and check the storage setup, especially on a dual-boot or virtual-disk system.

See the partition that carries each role

Run this third check if you want to see the individual partition numbers.

Get-Partition |
    Where-Object { $_.IsBoot -or $_.IsSystem } |
    Sort-Object DiskNumber, PartitionNumber |
    Format-Table DiskNumber, PartitionNumber, DriveLetter, IsBoot, IsSystem

This is the result observed on one Windows 11 PC, build 26300. The output below keeps only the columns shown in the command.

DiskNumber PartitionNumber DriveLetter IsBoot IsSystem
---------- --------------- ----------- ------ --------
         0               1              False     True
         0               3 C             True    False

The first row is the system partition and the second is the running Windows partition. They have different partition numbers on the same physical disk. The two-drive scenario in the first card is an example for comparison. The test PC had one disk, and the test consisted of reading Windows' storage information.

For a visual cross-check, open Disk Management and match the disk numbers in the lower pane. If you see more than one partition labeled EFI System Partition, use the PowerShell result to identify the one Windows marks with IsSystem. The partition label describes a type, so two copies can carry the same label.

Keep the old drive intact until a standalone boot works

If you are replacing a drive, back up its files and confirm that you can access the backup. If the PC is encrypted, have the matching BitLocker recovery key available. A Windows 11 recovery drive can help if startup fails, but your files still need their own backup.

After the startup dependency is resolved, shut the PC down fully and follow the device manufacturer's instructions for disconnecting the old drive. With that drive absent, start the PC and sign in to the intended Windows installation. If it fails, shut down and restore the original drive configuration. Keep the old drive intact until you have a verified boot and a copy of what you need from it.