Because they barely do anything. It's like like there's 4TB of RAM in there churning away at multiple databases. It's debatable if you even need it in enterprise servers.
You absolutely, positively, 100% need it on anything that carries data you care about. I personally consider it a hard requirement for a NAS. I don't want to lose data just because a cosmic ray flipped a bit somewhere.
There's a lot of places the file can get corrupted on its way to your drive. The memory of the NAS itself is only one of those. If you want any certainty you need to verify it after writing, so ECC RAM is not enough. And once you do set up that verification, you don't need the ECC RAM anymore.
I am not sure this is correct - there could be an error not only in the data, but also in instructions - but flip could cause data to be written to an incorrect ___location of the hard disk.
Possible, but I bet the risk of writing/losing the wrong sector based on that kind of RAM bit flip is a lot less than the other sources of writing/losing the wrong sector.
That differs based on the program you're using to put files onto the NAS.
But I'll note the even with ECC you need to double check things in case there was corruption on the drive wires or in many other places. With the right filesystem you can find some of those locally during a scrub, but double checking end-to-end isn't much harder.
Storing error correcting codes/checksums/signatures and sending them along the data seems like a more cost effective solution. Without those you need to ensure that every single link in the chain supports ECC (and all the hardware works perfectly)
ECC may still be needed for the actual processing, but I don't see a point on having it on a NAS (especially considering you need to send the data over a network to read or write it)
Absolutely. Imagine you are saving a text file to NAS with a super-secret password to your Bitcoin wallet, for example "password". While it was in memory before it reached disk, one bit was flipped and the file contents became "pastword" which OS happily saved on your RAID. And now you've lost your Bitcoins forever.
Consider all the RAM along a network transmission. Maybe you’re using authenticated encryption, maybe your transfer has an internal or out of band checksum. Maybe not.
