OK, I've noticed a pattern - I have drives setup as follows - #1 contains images from 2001-2014, #2 contains 2015-2019 and #3 contains 2020.
If I search across all 3 drives I get images in this kind of order : 2008, 2020, 2015, 2008, 2020, 2015, 2008, 2020, 2016 etc.
So it seems to showing images from Catalog#1, Catalog#3, Catalog#2..., 1, 3, 2, etc
Simon