Why can't I make this partition active on Windows 10?

Question

Here are the drives and partitions on my computer, according to Disk Management:

Some background: Disk 2, partition 1 (the currently active partition) was my system partition back when I ran Windows 8. When I got Windows 10 I got a new drive, Disk 1, and installed it there. You can see that Disk 1, Partition 3 is my Boot drive - that's my current C: drive, but for some reason it's not active, and the "Mark Partition as Active" command isn't available. In fact, there are no active partitions on the drive that I'm currently booted from.

Questions:

1. How is my computer booting from a partition on a drive with no active partitions?
2. If I remove Disk 2 from my computer (or if it fails), I will have no active partitions at all - will my computer then be unable to boot?
3. How do I make my current boot partition (or any partition on my boot drive) active?

Answer 1

Judging by the fact that your boot drive has an EFI System Partition, I surmise that the disk is GPT and your computer boots with the UEFI specification (instead of MBR with BIOS). That boot method needs no concept of an active partition. If you tried to make a GPT disk's partition active using DiskPart, you would get this message:

The selected disk is not a fixed MBR disk.
The ACTIVE command can only be used on fixed MBR disks.

Apparently, Disk 2 - which you used with Windows 8 - is an MBR disk. Your previous system booted using the active partition, but assuming your current bootloader is on the EFI partition on Disk 1 (you can check this with bcdedit /enum /v), the absence of Disk 2 won't affect your system's ability to boot.

Answer 2

As mentioned, this comes from GPT. You won't be able to set active partitions there, and when you try to activate the partition with diskpart, you will receive this:

The selected disk is not a fixed MBR disk.
The ACTIVE command can only be used on fixed MBR disks.

I am answering, because this was the top Google search result for this error message, and the accepted answer does not explain how to convert a volume to MBR.

You can convert your media to MBR using diskpart. This can be also helpful for USB boot media for older OS, if GPT does not work:

Attention: I assume the device is empty

diskpart
list disk
select disk #
convert mbr

(Note that you probably have to remove the partitions first.)

After this you should be able to create a primary partition and use the active command.