Is your SSD an NVME drive? It's possible that there are non-UUID references (maybe /etc/fstab, or GRUB's config) to the drive that are involved in the boot process.

Maybe it is looking for /dev/sda2, which is correct on the USB disk, but now everything is on /dev/nvme0n1p2.

Solution: Live disk, mount the root and boot partitions, look at the config files and fix the references.

-Or-

It could be that your boot manager has an an entry for the 128GB drive already, just pointed at the wrong .efi file.

If you were originally on Arch for example(, btw) on the 128GB drive. During the installation of the bootloader you would have inserted an entry into the boot manager like:

HD(1,MBR,0xe89l2937594, 0x3823, 0x2398734987)/\EFI\Linux\arch-linux.efi)

But now, since you're on Mint, arch-linux.efi isn't there and the boot manager falls over.

Solution: Live disk, use efibootmgr (https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface#efibootmgr), to delete the bad entry (\arch-linux.efi) and add one pointing to the correct file (\mint.efi ? grubx64.efi?).

e: It looks like Mint uses grub, so you could also live disk -> chroot into the environment -> run grub-install (https://wiki.archlinux.org/title/GRUB#Installation) to create the entry. You will still have a 'bad' entry which you can delete with efibootmgr.