Can we install a second internal disk drive?

Yes! If you have a SSD model you can add a second internal drive, the 2012 SATA cable sold by iFixit works just fine. You can see my upgrade at grayunicorn.com.

FWIW, OWC is about to release a kit that allows 2x SSDs in the Mac Mini 2014+.

Mac Mini 2014 is not worth upgrading. It's running an ultrabook processor and not a standard mobile processor. Even with the upgrades it is slower than 2011 Mac Mini. It cost an arm and leg just to get the SSD while the M2 version SSD that are standard cost 1/2 price and gets same performance.

"The tear down of the system has not been completed yet.

However if you go on there website it appears to be of the same physical size and shape, the power supply instead of being internal is now external.

They have an option for PCI express storage media which i would assume would connect VIA a M.2 slot which takes up less room than a 2.5inch SATA SSD. they also have an option for a single SATA drive.

That being said as long as when they do there tear down the mother boards are identical and the drive locations do not counteract each other than you have a M.2 PCI drive option with a standard either SSD or normal/ hybrid drive option.

My question is why would they remove the power supply unless they needed additional room? Is there still the option to have 2 internal SATA3 2.5 inch drives? And an additional M.2 drive?

For me that would be great I could get eventually a 1 TB drive that is M.2 sad and store the operating system(s) games and all software. in the 2 SATA slots I could install dual 1.5 TB drives which i would assume give between 80-135 MB/s speed with a combined data transfer in raid 0 of 150-270 MB/s depending on speed loss. Which is still fast enough for 1080p video and even fast enough for 2k or even3k and depending on how it will be compressed / data transferred it may be fast enough for 4k but 4k would push the limits of the drive.

I cannot wait for the tear down."