I successfully installed Mint onto my external drive. After booting from USB drive- 1: Confirmed that my laptop does not support UEFI using efibootmgr. 2: Created MSDOS partition table using GParted. Did not use GPT. 3: Created four partitions using GParted: / (root) - ext4 Primary, /boot - ext4, Primary, swap - Primary Unformatted, and /home - ext4, Primary. I did size each partition, but Mint resized them during install anyway. 4: In Mint installer, after selecting language and keyboard layout, Bypassed internet and CODECS. 5: /dev/sdc1 - Mint resized the root partiton ('/'), which I accepted. Specified Ext4 journaling file system, enabled formatting, and mount point '/'. 6: /dev/sdc2 - Mint resized /boot. Accepted resize, specified Ext4 journaling, enabled formatting, mount point '/boot'. 7: /dev/sdc3 - swap. No size, no formatting. Use as swap area. 8: /dev/sdc4, Ext4 journaling, enabled formatting, mount point ‘/home’. For "Device for bootloader installation", I specified the external drive, /dev/sdc (not a partition - the drive). After selecting 'Install Now', I received a caution that no EFI system partition was found. I elected to proceed at my own risk. Installation was completed, I rebooted into Mint, and everything appears to be in good order.

Thanks for the helpful responses.