This may be another way to do it. This is NOT a single file on an existing "disk". Instead, this is how I make a bootable "disk" as a backup.
First, you have a HDD with space on it, but also containing other files already. To make my process work, you would have to use Disk Management (or Acronis, or something similar) to SHRINK the existing Partition on that HDD to a size small enough to create a chunk of empty space that is Unallocated Space. That leaves all your existing data in the (smaller) Partition on that HDD. The aim is to create enough Unallocated Space that you can create in it a second Partition that will hold a clone copy of your current C: drive. That space does NOT have to be the entire size of your C: drive. It does need to be a little bit bigger that the actual size of the USED space in the C: drive. For example, if the C: drive is now 500 GB but it only contains 325 GB of stuff with 175 GB Free Space, then you need a chunk of Unallocated Space about 330 to 350 GB.
So, IF you can do that, then use a cloning utility - I have used Acronios True Image for this - to make a clone copy of your C: drive. In doing that, you need to tell it to create the Partition that will receive the copy in the Unallocated Space you freed up on your spare HDD, making it the size you want and NOT just the exact same size as the current C: drive. Ensure that this new Partition is designated as a BOOTABLE partition. Then let it make the clone, and back out of the cloning utility. Now the second HDD will have two Partitions on it. One will be your original Partition, smaller than it was before, but containing all your old data. The other will be a new bootable Partition that is a complete clone of your C: drive. Each of these Partitions will be treated by Windows as a separate "drive" with its own letter name and completely accessible under Windows.
The important part here is that the new "drive" (Partition, really) on that second HDD is also directly accessible to your BIOS at boot time. To prove this to yourself, shut down the system and disconnect your normal C: drive. Now power up and go immediately into BIOS Setup. In there, change the Boot Priority settings so that it will boot from that second HDD (now may be the only HDD in your system), then SAVE and Exit. The system should reboot, find the bootable Partition on that HDD, and boot from it. That Partition now will be called your C: drive by Windows (because Windows always labels the drive it booted from as C

. You also should see your original Partition containing data as another "drive", but of course your original C: drive will not appear because you have disconnected it.
Once you have succeeded in this process and proven that it works, shut down again. Re-connect your real original C: drive, then use BIOS Setup to restore the proper boot priority sequence and boot from it. If you ever lose the C: drive, you can connect things as was done in the test above and you have a bootable spare C: drive to work from until you can replace the old original C: unit.
At any time in future you can use Acronis to go back to that second HDD with the two Partitions on it and Delete the one containing your C: clone. Then you can re-do the process of making a new clone copy to a new bootable Partition created from that Unallocated Space, thus updating your backup clone.