All I wanted to do was boot from my trusty Clonezilla USB stick to make a system back up of my shiny new Thinkpad X260. Long story short, you need to disable the 'Secure Boot' option in the UEFI (what used to be called BIOS) to boot from a bootable Clonezilla USB stick.
I bought the X260 a couple of month's ago. This is last year's model, so I got it at a discount. Usually I'd buy a 2/3 year old Thinkpad and replace the drive and keyboard, but found I could buy a new laptop, albeit last year's model for about half of what it would've cost a year ago. A quick cleansing of the OS by installing Linux Mint. The usual kerfuffle to configure the system and remember how to partition the drive, then try to remember how to mount said partitions. Time to make an image of the OS partition. I've learned this is a good idea the hard way. When I tried to boot from my Clonezilla USB sticks, none of them would work! Somehow I had made a bootable USB stick that would install Linux Mint. I spent a good hour before checking on the 'Secure Boot' option in the UEFI screen, which by default is Enabled. Flicking this to Disabled solved this issue.
Now I have an image of my OS on an external drive for when I manage to destroy the installation. Not if. When. Still, that's how we learn, by breaking and fixing. Probably a good thing that I don't work in medicine.