Sorry for the delay in answering, but I was shocked that those instructions did not work, and have spent a very long time trying to discover why not.
braziers wrote:Out of interest, if grub doesn't even detect my usb then how is it going to find this file via this method?
That is exactly the right question, and is the heart of the problem.
I know it used to work that way a couple of years ago - my notes on how to do it are very detailed.
What I have seen today shows that Grub installed to the hard drive does not detect the partitions on the flash drive, as I would expect it to do. I even change the format of the flash drive to FAT32, and created more than one partition, with no change.
Then I flipped it around, and booted a USB hard drive (with the flash drive connected), and the partitions on the flash drive were then recognized, and the boot sequence that I gave you worked fine to boot my Mint xfce .iso
HOWEVER, none of my partitions on my 2 internal SATA drives were detected by the Grub on the USB hard drive. That mystifies me.
braziers wrote:Is there not a way I can partition the hard drive or something (via ubuntu) and access the ISO afterwards, through grub?
Yes, there is. I do that all the time. However, it won't work with your 20GB hard drive, with only 1GB of RAM.
I can see only 2 ways possible for you.
SuperGrub2 should work. I can't imagine why that failed for you.
Next possibility, take the hard drive out and install it in a different computer. Install Mint. Then re-install the hard drive in this computer.