So as you can see the compression results can vary significantly from one computer to the next.
Besides, creating a backup of system on internal hard disk is a risk operation; the backup might be lost due to hard disk failure, unintentional deletion, virus attack and other reasons. Brian K Jun 4, at 7:
Create ISO Image of USB Flash Drive
4 Portable Tools to Make a Full Image Backup of USB Drives • webforum.top
Create Windows 10 System Image on USB Flash Drive
You can copy the files from your USB drive to a folder on your computer and copy them back whenever you want to restore them to your USB drive. Log in or Sign up.
It is also provides a means of restoring Windows 7 or 8. This process will erase the contents of the USB drives entirely, overwriting them with the data from the image file.
How to Create System Image on USB Drive in Windows 10/8/7? – EaseUS
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The whole device will be backed up uncompressed to the IMG file format and the file can then be viewed or files extracted by tools such as 7-Zip or UltraISO etc.