Let's start with the question:
Is it necessary to house or enclose magnetic hard drives when they are running?
No, it is not neccesary. The drives will work as long as they as provided with power and with a data connection to the disk controller.
I'm building a DIY NAS, and I'm looking for a solution to physically arrange hard drives together. I've seen products like this, and I'm wondering if they are actually suitable for long term operation.
I know you're not suppose to run a magnetic hard drive just placed on a desk or something because the motion generated by the rotations might be problematic if it's directly absorbed back into the drive. Another thing a traditional case provides is shielding from dust from the environment; but I think hard drives are fairly shielded anyway.
According to this wikipedia article, high temperatures were not well correlated with disk failures in a large sampling. In addition, dust is common in almost all applications since, if the inlets are filtered, they are rarely done so with a filter that will filter fine particles.
So, I would say pick whatever enclosure (in no particular order):
Looks good (we like our custom builds to look nice)
Is easy to get the drives in and out of.
Provides a clean way to wire the drives.
Fits with your other components.
well thats a cooler, not a nas. a nas will contain a OS with a lot more features that you could need to run your nas. whats really important when having a nas is the hard drive, you want something reliable taht runs 24h without any problems. usually on the WD side, the red series will give you what you need – Fluffy Destroyer – 2017-03-17T21:18:47.037
Yes, I understand that - I'm looking for a way to arrange the hard drives safely physically and then I'll build the NAS components around it. – Nick Hu – 2017-03-18T13:41:42.803