The M4 Mac mini kept its SSD on a removable NAND module under its cover. The M6 and M5 Pro models solder the NAND to the logic board, and 9to5Mac mourns the hidden feature that gave the old model much of its value. Whatever capacity you pick at checkout is the capacity it keeps.
On the M4 model, per the video, an owner could swap a failed module at home or fit a third-party module of up to 2TB into a 256GB base model later. The how-to for that upgrade, shot on 9to5Mac's own M4 Mac mini, drew about half a million views by its count. For the M6 and M5 Pro models, its line is "it was nice while it lasted."
The 256GB M6 model also runs on a single NAND chip, which the video doesn't mention, and its Disk Speed Test scores came in at roughly half those of the 512GB and larger models, per a MacRumors report. With soldered storage on top, I'd treat 512GB as the real starting point for iPhoners buying an M6 Mac mini.
The M6 Mac mini reached Apple Stores on September 22. For a base model already bought, the video's fallback is a fast external SSD on Thunderbolt or USB-C; for anyone still shopping, it's a used M4 model that takes a module.
