Why is a separate partition marked as active instead of the boot drive

I have two storage devices in my PC. One is a 128GB SSD which is used as the System disk (where I installed my OS) and the other is a 1 TB HDD which is used for keeping my usual files, docs, movies etc.

As I have mentioned, Windows 10 is installed in the SSD, I never installed any OS on my HDD. So, why is "E" marked as Active and contains boot files?

Did you have the second drive in when you installed Windows? A quick test is pull the power from second drive and see if the ssd will still boot.

The SSD can not boot by itself, because the partition is not marked as active.

diskpart
select vol C:
active
exit
bcdboot C:\Windows /s C: /f all
exit

NavyLCDR said:

The SSD can not boot by itself, because the partition is not marked as active.

diskpart
select vol C:
active
exit
bcdboot C:\Windows /s C: /f all
exit

I did that. The boot drive was swapped but "E" still remains as System partition

Now you need to change the boot order in your BIOS to boot from the SSD (Disk 0) first. Alternatively, you can also try unplugging Disk 1 (the HDD) and see if the computer will boot without it. If it does, then you can remove the active flag from that partition.