I'm pretty sure the fact that the new Mac mini uses the T2 chip (which among other things is the SSD controller with 256bit encryption) that the SSD has to be one of Apple's and most likely soldered on the board like the MacBook, MacBook Pro and the new Air...
Although the Mac Pro (trashcan) does also have a T2 and it might have upgradeable drives. (Sorry, too lazy to go look right now.
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Although the Mac Pro (trashcan) does also have a T2 and it might have upgradeable drives. (Sorry, too lazy to go look right now.