Upgrading a Late 2014 1TB (non-Fusion) Mac with a SATA SSD.
An Dinh -
My mini was slow. I purchased the mid-grade model with the upgraded CPU & RAM (2.6GHz i5/8GB) because those things are not upgradable on this platform, even if they were, a CPU would cost a 1/3rd the price of the computer so, I skimped on storage purposely to save cash.
This left me with a 5400RPM disk with cutting edge performance from 1998.
Pretty quickly. I chose to go with a 500GB Samsung 850 EVO SSD.... This was ridiculously easy with iFixit's 64-bit kit (TR6 bit) & the logic board removal tool. The teardown was useful but, this is a simple upgrade.
Unfortunately, in order to take advantage of Apple's PCIe Blade SSDs, you have to buy a ridiculously expensive (& used) unit from eBay. Then, you'll need the caddy which will allow you to add the Apple SSD along with a SATA HDD to create your own Fusion disk.
The ideal route would be to grab the caddy, an Apple PCIe SSD (128/256/512GB,) AND a 1TB SATA SSD like the Samsung 850 PRO/EVO models. This would result in the fastest performing disk I/O any Mac mini has likely ever seen.
I did not clone my disk, I did a fresh install of 10.11.3, then used Migration Assistant to pull my user information & files off the old disk via USB. I prefer it this way because I like the idea of a freshly installed OS... there is nothing wrong with cloning, this is just the route I chose.
You can find tons of information about the mini and the disks it supports from Mac forums. I suggest doing your proper research before you order anything, as these upgrades can be costly.