My question: Can Gnome utility create an image of complete HDD or SSD irrespective of whether HDD contains partitions or not? For example I have a HDD which has three partitions , can I use Gnome to create an image & then restore it. I will use a bootable Linux USB for this. thanks
Gnome Disks includes the option
Create Disk Image. This does just that: make a file of all numerical data on the disk. This includes all partitions on the disk.
You intend to use a "bootable Linux USB". It does not work this way. You restore the image to a disk, which can be an USB disk. Its contents will be fully overwritten by the disk image, so whether you use a "bootable Linux USB" or any other USB drive with sufficient capacity does not matter.