Does booting a compressed disc decompress it?

If a compressed image of my HD on a disc is loaded into my internal HD will it reside there in compressed form or will writing to the PC's internal HD decompress it?

Restoring a disk image to another drive will automatically decompress it as it is written. Copying an image to another drive is just that.. copying it.

If you are just copying an .img file then no it will not "decompress" not untill you mount the .img then it will be accessible. The disk .img will stay as a disk .img no matter how many times you use the .img to boot from.

I have two contradicting ideas about what happens when I use an imaged HD to reboot with. One says the data on the imag disc is not decompressed as it goes to the internal HD and the other idea says it is. Have I misunderstood somewhere? And are you saying I can use an imaged HD to reboot from? If so why do I need a separate reboot disc?

I'm sorry I misunderstood. I thought you meant you have a copy of Windows as a .img file on a disk and you boot up cpu thought this .img. to properly answer your question. If you just copy a compressed file or what ever it is. It will not decompress. It will just stay in original form in till you open it etc

So have I got this right? I have an internal HD which is not compressed. I make an image of it and compression occurs on the image. I then copy it back to the internal HD and it does not decompress until it is read?

Depends what software you are using. The image will be the full size of what ever is in it. You can use software that compresses a file and puts it as a .img in one go but like I say it depends what your using it to put it as an "image" file

---------- Post added 14-01-2017 at 22:16 ----------

To be honest I don't know what you are trying to do. You want a copy of your hard drive as a compressed .img to save on said hard drive? Are you meaning "image" as in a mirror image "copy of everything" cos image in cpu speak is a mountable .img you can use for anything Such as games, programmes, applications, full Windows set up etc etc

If you write the image back to a HDD using the imaging software, it will be decompressed, as the whole point is to put the image back just as it was on the original HDD.

If you are accessing/mounting the image file rather than writing it back to the HDD, it will decompress as you access it.

If you copy the image file using normal filesystem tools such as explorer, then obviously it will remain compressed, its just a file.