Yes, I’ll try to do that.
Try booting that NET installer USB. When you get to the boot screen, there should be an option for “Rescue syste”. You may need to scroll down to find it.
Boot to the Rescue system. That should eventually get to a login screen (command line login). You enter “root” for the login. It won’t ask for a password.
Once you have have logged in a root, run:
fdisk -l /dev/sda
and post the output here.
For small amounts of output, maybe pencil and paper works. But it is best to post to a file. For that, I often plugin another USB with a FAT file system. And that might show up as “/dev/sdc”. So I would mount it with:
mkdir /a
mount /dev/sdc1 /a
And then I can create files there. So I could use:
fdisk -l /dev/sda > /a/fdisk.out
and that should put the output in the file “fdisk.out” on your second USB.
Leave that second USB plugged in until you shutdown or reboot the system, so that it is cleanly unmounted.
Unfortunately, systems are not all the same. So maybe the second USB isn’t “/dev/sdc”. I don’t know enough about your system to do more than crude guesses at the moment. So do your best, and report back. And what you report back will help next time.
Actually, when in rescue mode, try:
ls /dev/sd?
then plugin your second USB and repeat the command.
The output from those two commands will be pretty small, so that pencil and paper will work. And post that, as it will give us information about your system for the next round.