It involves raw disk access, ignoring all partition layout -- just starting at sector zero and going all the way to the end. In Linux this is easy, in Windows it's a pain.

If you read the data from a compact disk beginning to end this way, and saved it to file, you get an ISO file. No translation is necessary, the data started that way.

Code:

dd bs=4M if=/dev/cdrom of=myfile.iso

You may need root access to get at the device.

If you read all the data in a USB thumb drive from beginning to end and saved it to file, you'd get a disk image:

Code:

dd bs=4M if=/dev/mythumbdrive of=myusbdrive.usb

You will definitely need root access to get at a raw disk device.

What "kind" of image depends on what it was formatted as. Unlike a CDROM, USB storage devices are just raw block devices which can hold anything and everything. The only difference between a disk formatted as GPT and one formatted as a Solaris partition type is what data is contained in those raw blocks. If you back up the contents, you've backed up literally everything. There is no difference between a thumbdrive image, a hard drive image, an SSD image -- it's all just blocks which make as much sense on any of them.

To put an image back into the device, you'd reverse the process:

Code:

dd bs=4M if=myfile.usb of=/dev/myusbdrive

You will definitely need raw disk access to do this, which is dangerous, because:

Moderator's Comments:

UNIX WILL NOT STOP YOU FROM TRASHING YOUR HARD DRIVE IF YOU GIVE THE WRONG DEVICE. UNIX WILL NOT STOP YOU FROM TRASHING A DEVICE THAT'S IN USE. BE VERY VERY SURE WHICH DEVICE IS WHICH AND THAT YOUR SYSTEM HAS NOT MOUNTED IT.

Giving more specific instructions would mean knowing which linux. if it's ubuntu a lot of effort may be needed to defeat the automounter, etc.