back up iPhone to an external disk

If you create the backup using iTunes/Finder, I believe you can move it to an external disk, or perhaps you're able to change the location of the backup before it's created? It's been a while since I used my computer for backups. I've been using iCloud for years.

I found this user tip that may help. The link should take you to a section that provides instructions for moving the backup location.: Make a split library portable - Apple Community

~Lyssa

I’ve bought a portable solid state drive to back up my iPhone 12pro max whilst travelling. How can back up?

You cannot backup your phone to an external hard drive directly. You must use iCloud or iTunes/Finder on a computer.

~Lyssa