Sorry about the length. Once I started, I realized that some of this was hard to describe. Pictures would have been easier, but I’ve not got that system anymore so I had to go with words instead.
Here’s what I did for the 7010 Motherboard. Probably not the only way, but its one way that works, it boots without error messages, and requires no cutting.
A note up front: as with many transplants, there’s some feature loss. In this case, you loose the front panel connectors (USB and Audio), and some (or all depending on your case selection) of the front side indicator LEDs (Power, HDD Activity, etc.). Probably not much of an issue in a NAS though. You also won’t have an I/O shield but that’s also pretty common for transplants as well. In my case, I trimmed an old I/O shield to fit. But, but as you’ve probably seen from the forums, many folks just do without.
Motherboard: The mounting holes have standard spacing. Its as deep a a Micro ATX, but just a little bit wider than ITX. I had no issues using a deep ITX Case (Cooler Master Elite 130) for one of the two builds I’ve done with these motherboards. The power connectors are also standard, so you can use the power supply of choice. As you note above, the proprietary Dell PSU won’t fit in a standard case, and its not worth modifying it.
Power Button: This part was hardest to describe, so if it makes no sense, let me know and I’ll try again… Dell’s power button cable has a proprietary (non-standard pitch) connector on the motherboard side and some proprietary foolishness inside the wiring that throws a warning message if you try to use a generic button assembly. Luckily, the square button housing on the other end of the cable assembly is a common size used in a lot of cases. So I was able to simply swap Dells’s power button assembly with the power button in the case(s) I used. You want to find a case that doesn’t have a separate plastic button cap that fastens to the switch itself (hope that makes sense?). A lot of cases now have “power buttons” that are simply a section of the plastic fascia that bends under pressure and pushes a switch mounted to the metal frame beneath. When you pull the plastic front piece off, there’s no switches hanging off it. That’s the kind of case you need. If you want a visible Power LED, you need to pick a case with a power button that lights up (like the Dell’s does). A last consideration is that the Dell switch assembly cable isn’t super long. So power button placement on the case is a factor (unless you want to splice extensions on the power button cable)
Here’s a couple pictures of a Cooler Master Elite 130. I used this model on one of the builds. Power “Button” in the first picture is just a strip of plastic that bends and contacts the switch underneath. The LEDs in the switch housing shine though the plastic. The actual switch snaps into the metal frame (white square on the right side in the second picure).
Other front panel connectors: If Dell’s proprietary front panel connector assembly (USB, Audio, and some LEDs) isn’t plugged in, the system will give a “Press F1 to continue booting” warning at boot. I took the assembly from the Dell case (a couple screws), plugged it into the motherboard in the new case and zip tied it inside the case in an out of the way place (The unused 5.25 drive bay in the ITX Case, and on the motherboard tray in the full size ATX case). You’ll still have USB and Audio on the back I/O of course.
Dell’s Case Fan: To keep from getting a “Fan Missing” message, I simply reused the Dell case fan in my builds. It was all I needed for the ITX case. For the ATX tower, I added some low RPM 3 pin Noctua fans and powered them off the power supply with Molex to fan adapters. Its possible to get adapters for Dell’s proprietary 5 pin to standard 4 pin PWM fan plugs, so you can use non Dell fans. The problem is that unless you use a fan that matches the original Dell fan’s power draw and RPM range, the system will give you a “Missing Fan” message. I tried a lot of fans and none “matched” what the system was looking for, so I found it easier to just reuse the Dell fan.
As for OS: I’ve used all three that you’re considering. I like Unraid best. If you are familiar with Linux, you can get much the same results with OMV… and I have. But, OMV requires a lot more tinkering than Unraid to do it. FreeNAS has great enterprise features, but I never used most of them, and it wasn’t as tolerant of my energy saving steps, like drive spin-down. Like a lot of Home NAS, mine is idle most of the time, and with modern drives rated for 1/2 million or better up/down cycles, its worth it to me to have drives spun down when its for hours at a time.
Concerning your hardware selections: That’s pretty personal and I’ve got no direct experience with either of these platforms on the desktop side. I will say that all three NAS OS options can be “behind the times” in terms of hardware support so its good to check the various forums and see what’s causing problems if you want to go new.