The Mini 9 takes a specialized SSD
. Fortunately, the Mini 9 was popular enough that many different manufacturers made them. Unfortunately, some suck. And also, few retailers carry them these days. And they tend to be overpriced, compared to other SSDs and other flash storage.
But they're the only bootable drive available for a Mini 9, aside from a USB drive (which has it's own drawbacks).
Try searching the web for "mini 9 ssd
". I think the highest regarded choices are the RunCore brand SSDs and the second-gen SuperTalents (the ones with a GFDL suffix, I believe). You can also look for a used drive on these forums, or on eBay. Many of the used drives will be STECs (the OEM Dell drive).
An entire used Mini 9 could be another choice, too. Take the best battery, SSD
, etc. from the pair and make a Frankenstein machine