So if not a raid, and if the disks are external ( say in basement) then what is their home? Is it a Windows based OS?
It depends on your intended use. You might do external drives directly attached to your HTPC. If that takes up too much space or is too noisy, you could use a NAS box of some sort. I don't have direct experience with FreeNAS or any of the other DIY solutions. Though I hear some of them are good.
My main idea in all of this is to keep it simple at every step.
And how do I manage a current location library of 12 disks? Do you suggest some sort of pooling software?
That's a lot of disks! If it were me, I would be looking to move the collection to the smallest number of disks possible with (at least) two considerations:
1. Keeping 10 - 20% free on all drives.
2. Anticipating growth. If I expect to grow my collection, I'd try to buy drives large enough to account for that growth. If possible.
The good news is that MC can handle a collection that's spread out among many disks pretty easily. It just becomes a bit of a hassle to try to figure out which disk(s) have free space on them. I would probably approach this problem by filling each disk to it's intended capacity (usable space -10%). That way any additions go to the last disk which still has space left on it.
Pooling software is something I'm familiar with, but I'm not sure what the reliability of various systems is. So, as a "keep it simple" measure, I would avoid them until doing enough research to be really sure about what happens when there are drive failures. Also research how often these systems cause loss of all data in the pool. I wouldn't do it at all until I was really sure.
And would this be left on 24/7?
This is a hard one. The spin up and spin down of disks is bad for them. Disks that run all the time generally last longer. But that's ONLY if they are adequately cooled. For many years consumer drives ran "too hot", mainly in external enclosures. So manufacturers started adding spin down to the external boxes to keep the drives off when they weren't being used. This seems to mostly have worked pretty well. My personal experience with external drives has been pretty good over the past 5 years. I've lost one of 5 drives in that time. The oldest drive is now at least 8 years old.
So it's a crap shoot. The old advice is: "all computers and drives last longer when left on 24/7". But drives are a high heat device, so it makes that advice harder to follow unless the drives are in enterprise class enclosures with great cooling.
I personally think you could go either way with externals, since they will probably be doing "auto power" internally and shut the drives down when not in use for a while.
Again, if I were you, my first thoughts would be towards minimizing my number of physical drives and, at the same time, getting a backup strategy going ASAP. 12 drives have a pretty high probability of one of them failing.
Good luck!
Brian.