By deleting the Mint partition, you have basically deleted grub. Thus, when the machine code inside the MBR tries to locate the second-stage bootloader (GRUB) it will come up empty handed because the partition on which GRUB was located is no longer.
The failure of Unetbootin is, at least AFAIK, unrelated. A few checks: did you turn SecureBoot off, in the BIOS? Have you activated booting from the USB stick, again in the BIOS?
Alternatively, it is known that Unetbootin fails, occasionally. For instance, this Arch Linux Wiki page states:
UNetbootin can be used on any Linux distribution or Windows to copy your iso to a USB device. However, Unetbootin overwrites syslinux.cfg, so it creates a USB device that does not boot properly. For this reason, Unetbootin is not recommended -- please use dd or one of the other methods discussed in this topic.
So, if you are using another Linux system to format your USB stick, I suggest you use the following command:
dd bs=4M if=/path/to/mint_image.iso of=/dev/sdX && sync
Careful: sdX
is your USB stick. You must determine the drive letter appropriate to it, generally you cando this with
fdisk -l
The dd
command will wipe the contents of sdX
, so please make sure you are writing to the right device. Also, notice that it is sdX
, not sdX1
or whatever: it is the physical name, not the partition, that must be used. Lastly, the USB must be unmounted. If your OS automatically mounts it, then unmount it (without removing it, of course) by means of
sudo umount /dev/sdX1
where this time you must use the number after sdX
.
If instead you are using a different OS to format the USB stick, the above-referenced Web page has instructions on how to do it without using Unetbootin, both in Windows and in Mac OS.