I haven't studied the Mint installation image in depth, but the following should
- Partition a USB flash drive so that it holds one big FAT filesystem. On an MBR disk, give it a type code of 0xEF in fdisk (I don't know of a way to do this with parted or GParted); or if you use GPT, give it a type code of EF00 in gdisk or set the "boot flag" in parted or GParted. (Note that the GPT "boot flag" is entirely different from the MBR "boot flag," so setting the "boot flag" in MBR will not have the same effect.) On many (perhaps most) EFI computers, either MBR or GPT will work, but it's possible that on some, only GPT will work.
- Mount the filesystem you've just created somewhere handy -- say, /mnt/usb.
- Mount the Mint image file as a loopback device -- say, via "sudo mount -o loop mintimage.iso /mnt/cdrom", changing the image filename and mount point as necessary.
- Copy all the files, as in "cp -r /mnt/cdrom/* /mnt/usb"
- Unmount the image file and USB flash drive.
The flash drive you create in this way ought to boot as far as GRUB, and probably at least start to load the kernel. Beyond that I'm less sure; it's possible that the initial RAM disk will look for a filesystem with a particular volume label or UUID to load. If so, you may need to adjust those details, and I haven't looked into them in enough depth to be able to tell you what to do. You could try giving the FAT filesystem on the USB flash drive the same name that the ISO-9660 filesystem has, though.
There is another approach, but it requires two USB flash drives (or a USB flash drive and an optical drive -- but I assume you don't have the latter, or you'd just use it to burn the Mint installer):
- Copy the Mint installer to a USB flash drive by typing "sudo dd if=mintimage.iso of=/dev/sdd", changing the name of the Mint image file and the target device name (/dev/sdd in this example) as appropriate.
- Download the USB flash drive image version of my rEFInd boot manager.
- Unpack the rEFInd image, as un "unzip refind-flashdrive-0.6.5.zip"
- Copy the unpacked rEFInd image to your second USB flash drive, as in "dd if=refind-flashdrive-0.6.5.img of=/dev/sde", again adjusting filenames as necessary.
- Leaving both USB flash drives plugged in (or moving them both to your target computer), reboot and hit whatever key(s) you need to use to get to your computer's built-in boot manager.
- Select the rEFInd flash drive as the boot device. (It will probably be identified by device make and model, with "EFI" or "UEFI" in the description.)
- When rEFInd appears, you should see an entry for the Mint USB drive. Select it and the Mint installer should launch. (Actually, you'll probably see two entries -- EFI/BOOT/BOOTx64.EFI and EFI/BOOT/grubx64.efi. I'm not sure which one is most likely to work.)
This method has a chance of working because rEFInd includes an ISO-9660 driver that enables the firmware to read the ISO-9660 filesystem on the USB flash drive. This driver is the one thing that's missing from the firmware to enable it to boot the Mint image when it's written to a USB flash drive. Really the only thing you need rEFInd for is to load that driver; but since rEFInd launches in the process, it presents a boot menu that should let you select the Mint installer. If you don't see the Mint installer, wait a second or two and press the Esc key.