Interesting that Nautilus file manager sees all the drives but the df command sees only Mark's drive.  
Looks like you'll have to attempt a Grub repair after all. Perhaps Rich can help with that.
This URL looks like it might help with a repair: https://www.tecmint.com/rescue-repair-and-reinstall-grub-boot-loader-in-ubuntu/. You'll need to scroll down to "Reinstall Ubuntu GRUB Boot Loader".
I found here t...
	
				
			
	
Interesting that Nautilus file manager sees all the drives but the df command sees only Mark's drive.  Looks like you'll have to attempt a Grub repair after all. Perhaps Rich can help with that.
This URL looks like it might help with a repair: https://www.tecmint.com/rescue-repair-and-reinstall-grub-boot-loader-in-ubuntu/. You'll need to scroll down to "Reinstall Ubuntu GRUB Boot Loader".
I found here t...
Looks like you'll have to attempt a Grub repair after all. Perhaps Rich can help with that.
This URL looks like it might help with a repair: https://www.tecmint.com/rescue-repair-and-reinstall-grub-boot-loader-in-ubuntu/. You'll need to scroll down to "Reinstall Ubuntu GRUB Boot Loader".
I found here t...


