I need help interpreting the 3-2-1 rule...

I know. It seems pretty straight forward: 3 copies of the original file; 2 different storage devices for the data; 1 cache moved off site

The snag that I'm hitting is the 2nd verse.

Does that rule mean i have to keep my copies across 2 different types of storage devices? (Ex. Using an external SSD, and using Blu-Ray CDs)

Or does it mean I can use the same device type, but i need 2 of them. (Ex. Having 2 external SSDs)

Edit: Thanks for all the help!!