Only way I know how to do this is close to what @Maximus put but do the following:
diskpart.exe
select volume 1
detail partition
The output will indicate
Active: Yes/No
For example:
You have to go through each volume to find which ones are Active and which ones are not. You can use Diskpart's list volume
What you need is partition management and cloning software. Personally I use Paragon Hard Disk Manager (which would make much of what I'm about to say much easier... but it's not cheap). You can find free solutions such as GPartEd and clonezilla.
In your case you appear to already have enough free space available on Disk 0, so you only need clonezilla. Using that you can make a copy of the first partition from Disk 1 to the unallocated space on Disk 0. Understand that the names and even the order the drives are listed under clonezilla will be different. But you should be able to identify which is which by their dramatically different total capacities.
However, that still will not let you boot when you remove Disk 1. This is because there is a special bit of code found outside the partition in a place called the "boot sector". This bit of code is necessary for your BIOS to find the windows 10 boot loader and begin booting windows.
But before you solve that problem there is yet another: The active partition only indicates the boot partition for that drive. Each drive can have it's own Active partition. The point is that the "Active" partition does not, tell the BIOS which drive to boot.
To tell your BIOS which drive to boot you will need to go into your BIOS and check your boot settings, making sure your system drive is the first in the boot order. In fact, the reason windows may have installed it's boot files to the second drive is that it may have detected that this is your currently configured boot drive.
Once the BIOS is booting the correct drive you can solve the missing boot sector code by booting from your Windows 10 DVD and entering the recovery environment. Then run the "Startup Repair" option. This should find the missing boot sector code and create it again on your currently configured boot drive.
One last detail. Understand that your system appears to be using the older BIOS boot method. However, EFI is the default with fresh installations of Windows 10. As such most of the information out there about solving this problem will be focused on EFI and will not be relevant to you. Keep this in mind.
The answer is by @pat2015 :
Interestingly I find that the mounted EFI drive is accessible from the ‘Browse’ button from Task Manager -> ‘Run New Task’ You can then perform basic operations possible with Context Menu (Copy/Paste etc) and even when you run other apps with Admin Privileges e.g. Notepad.exe it can also access the drive from its file menu. The issue seems to be with Explorer.
The partition flagged "active" should be the boot(loader) one. That is, the partition with BOOTMGR (and the BCD) on it.
On a typical fresh Windows 10 installation, this would be the "System Reserved" partition, yes.
Of course, this only applies to MBR disks (booted in BIOS/CSM compatibility mode). GPT disks should instead be using a EFI System Partition, identified by the partition ID rather than any "active" flag. Windows can only boot GPT disks in UEFI mode.
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