There are now 2TB drives that are in the 2.5in range. (This is the size that goes in the Mac mini.) The difference is that the 2TB versions of these drives are 12.5mm tall.
We’ve done some testing though. You are able to put in a 12.5mm drive if you use the upper bay of the Mac mini. When it is installed there, you aren’t able to use the screws but it’s large enough to stay in place on it’s own.
Once the 2TB drive is installed, you’ll have room to put a 7mm drive in as well. Most SSD drives are 7mm including these excellent Samsung SSD drives.
When all is said and done, you are able to have a 2TB HDD and a 1TB SSD. That’s a ton of space for a great little server.