Trying to set up Mint for a friend on his PC. I installed it on an external hard drive. When I plugged it into his PC, it isn’t showing on the boot list but it shows on mine which is confusing me. I tried to disable the secure boot but still, nothing. I am not sure why the external drive isn’t showing on his PC.
What kind of PC is it? And what size is the HDD?
Do you have another drive? It can be a defect of the drive you are trying to use. I have had this happen with a few lesser-known brands. For whatever reason, not all computers read them.
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Agreed @nightstar I was playing around with XPE, and my USB drive was bad. I had to get a Samsung brand USB drive, which did the trick 
Puppylinux.com FossaPup or other live linux media on a usb flash drive. Boot puppyup from the USB flash drive and look around with the gparted application already an app inside puppylinux disk. gparted.org I assume the Linux Mint did all the work and selected the GPT over the MBR (Master Boot Record) type partition table.
I believe the answer is that MS Windows™ is blind and racist, ie unable to see other races of operating systems. ie disk partitions and file systems formatted on those disk partitions. Dumb question, Did you format your external Linux Mint hard disk drive with partitions and formatted those partitions in FAT32 0B,0C or NTFS 07. Well that is the only 2 partition types and formatted files systems that Windows natively reads. WITH Windows Subsystem for Linux 2 (WSL2), Windows might be able to mount other partition types and read other file systems.
Of course for Linux Mint install you use Linux Partition type 82 and Linux Swap partition type 83, and windows™ is blind and can not SEE those partitions. They don’t exist for Windows. Table of Partition ID type numbers. Partition Types ID numbers
MBR versus GPT label Partition
For newer computers built since 2010, select GPT style
Partition Types ID numbers with explanations and references.
Fred, Hope that is enough URL links to explain it all.
When I plugged it into his PC, it isn’t showing on the boot list but it shows on mine which is confusing me. I tried to disable the secure boot but still, nothing. I am not sure why the external drive isn’t showing on his PC.
Check if the BOOT and ESP flags are set for that partition on the external hard disk? Yes, disable “secure boot”. Might be the BIOS Boot Order list. Put USB Mass Media before hard disk? Use the key to select the “boot drive”. Yes, your problem is confusing, I agree. Did your friend finally able to boot Linux Mint?
Welcome to our tech community! Thanks for taking the time to join our discussions. 
I would advise you check if your friend’s PC can utilise either UEFI or Legacy BIOS to boot. These two boot modes can help solve the puzzle. For UEFI, you would have to manually disable Secure Boot. The USB drive should then show among the UEFI boot options.
For Legacy BIOS, just manually turn on USB booting in the BIOS settings.
To do all this work you need to install the ISO file first of all. Then you need to do rufus MBR or GPT mode depending on your device. It works perfectly either for Windows or Ubuntu release. Because with Assistant manager of Windows it could face a problem and having flash defected in the middleware like when attempting to install Windows 11 on a flash and an error of software let flash defected as error occured and can no longer flash it.