I'm running Windows 10 v2004. I just installed a new M.2 SSD drive (F) that is faster and larger than the drive my C drive is on. I have cloned C onto F (using Macrium Reflect Free) because I want to boot to this new drive. So my understanding is that I can now change the boot sequence in BIOS and boot to my F drive.

The problem is that I can envision problems occurring when what windows and all apps believes is C is now actually F. Program files, registry settings, applications, who knows all what. So to remedy this I would like to do the following:

1. Reboot, change boot sequence to boot from F. (How will I know it actually booted to F and not C, since the 2 drives are identical??)
2. Change the drive letter of C to H (first unused drive letter - will it allow me to do this??).
3. Change the drive letter of F to C.
4. Reboot, change boot sequence to boot from C again (which is the new M.2 SSD drive).

Is there any danger in all this? Would you expect it to work correctly? How can I know definitively that Windows booted to the expected drive? When I change drive letters will it effectively trick BIOS? Or will BIOS track this change, and change its boot sequence to match?

Thanks in advance.

1 Answer

When I change drive letters will it effectively trick BIOS? Or will BIOS track this change, and change its boot sequence to match?

No. Neither. The BIOS does not know what a drive letter is – it's a completely Windows-specific concept and is not actually reported to the firmware nor even to the drive itself. (It is only stored in the Windows Registry.)

Usually the BIOS boot order is only based on physical ports. Meanwhile, UEFI boot order is based on partition UUIDs that are stored within the disk's GPT header. (I don't know how your firmware would deal with duplicate UUIDs after cloning.)

If you ever see Windows-like drive letters in the BIOS UI, that's only because your firmware tries to mimic the default drive letter assignment that DOS and Windows use (e.g. first partition on first ATA port gets 'C:' and so on), but it does not mean that the BIOS actually knows anything about your Windows configuration.

Is there any danger in all this? Would you expect it to work correctly?

In my experience, cloning only goes well if you physically disconnect the old drive when booting the new one for the first time. This somehow causes the 'new' Windows to refresh its drive letter assignments in Registry, mapping its new boot partition to the C: drive. After the first boot is done successfully, the old drive can be reconnected and its "old C:" partition will get assigned a new free drive letter.

If you don't do this and try to boot from the new disk... Windows will still have C: mapped to the same physical disk and partition as it did before, and yes, in some cases it will indeed end up using files from the old disk. 0/10 would not recommend.

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