How does a disk image work? Why does Windows fail to mount an image?

Question

I'm having trouble mounting an image file in Windows 10 and I would like to know if my understanding of a disk image is correct. I hope this isn't a silly question and if there is some flaw in my thoughts, you can help me figure out what it is.

Disk image

As far as I'm aware, a disk image is an accurate bit by bit copy of the data stored on a physical drive. I have created my own disk image of the SSD in my computer. It has exactly the same size in bytes as the SSD which absolutely makes sense to me. This means there is no additional information (such as meta-data) stored in the image file, other than the exact copy of my SSD.

So if this is true, I think it wouldn't make any difference which software is used to create the disk image. Wouldn't the result always have to be the exact same file, of the exact same size independent from which program is used? Or am I wrong?

Unable to mount

Then how can it be that Windows is unable to mount the image? Since there cannot be any differences in the file format depending on a specific software used. It is exactly the same data as on the SSD that Windows was indeed able to read from. Why can Windows not read that data when it is stored in an image file?

I have multiple ISO files lying around. Windows can mount some of them and others not. Why is that the case? Isn't it all the same file format?

This error message pops up when Windows fails to mount a disk image: Screenshot

Couldn't Mount File
Sorry, there was a problem mounting the file.

None of my image files is broken. I can open all of them with 7-Zip, extract files and it works perfectly.

Thanks for helping me to shed some light into the dark.

Answer 1

There are many tools to make a complete functional disk image, but not all tools make a true clone of the original disk. For example, Macrium Reflect can make a compressed image, which takes up less space. It also has Intelligent Sector Copy, "Only backup the sectors that are being used by data on the disk. Pagefile (pagefile.sys) and hibernation (hiberfil.sys) will also be excluded."

That said, most such tools enable mounting of even compressed images through that specific tool. Using Reflect as an example, again, you can select and mount an image. Files may be browsed and programs may be run from that mounted image; one can even access locations that the OS normally blocks.

It is also possible to make a true, bit-by-bit copy, which might be used for forensic purposes; such an image could be restored directly to another disk to make a clone. For most purposes, though, this is inefficient in time and disk space.