Not a problem but an upgrade. A friend of a friend was in the market for a new computer and I convinced him to buy a Mac. A large selling point was the ability to purchase an older Mac and install a second drive and install Windows on it in the off chance he'd ever want to go back to it.
"Repair" itself went along fine. I had previously placed an SSD in a 2014 Mac Mini so was familiar with the system. The 2011 I worked on this time was nearly identical except I was putting in a second drive which is much easier in the older models of Mac Minis. It was as easy as the 2014. Ample time was set aside for the "repair" and I followed the step-by-step on ifixit.com very carefully.
Working on a Mac Mini, ifixit's guide says to be aware of the holes on the inside of the case that the HDD fits into while putting back together. I cannot help them stress this enough. Inside the system is tight and if the pegs on the drive aren't seated perfectly in these holes everything won't fit well enough. Beware the rubber washers on some of the bolts. These are amazingly cool to keep everything held together and buffer parts from one another but if they are forced even moderately they will shred from the friction and be useless. This is especially important when placing the antenna grate at the end of installation if the HDD is not seated perfectly.
Advice not pertaining to the actual repair but to the use of a second HDD to install a second OS on. It was the most difficult part of the process. Via Boot Camp I found it IMPOSSIBLE to install Windows on the second drive. I've built many dual boot machines and this is the first time it was a problem. It seems Boot Camp has an issue installing Windows (in my case, unsure if it's true for others such as Linux) onto a second installed drive (without a faster port, it can't be done from an external at all). I attempted it with this Mac Mini and a Mac Pro and was unable on both. The caveat is, I think, is that ONLY the main drive can be partitioned and Windows be installed on that partition. When I attempted it there was no issue at all. The friend I was doing this project for only had a 500 GB drive and his personal data left him with little space on it, leaving little expandability for his Mac much less a second OS on the same drive. It was the whole reason for installing a second drive! In the end, I had to partition the second drive in a Mac Pro, stop the installation process from Boot Camp Assistant, remove Mac OS drive, place the now FAT formatted drive into slot 1 and boot up from Windows install disk, install Windows as if it's a PC, then install that drive into the Mac Mini's second hdd slot to get it to work. Horrible to do the same repair twice but it was a learning experience.