Thank you, everyone, for the many helpful replies.
OK, moral of the story, backup your NAS. Absolutely. But I wonder if this isn't just shunting the problem down one step. 'Cause with that, now I have ext4 files backed up to compressed files on yet another box (since clearly I'm not backing up back onto the hard drive of my PC.) Cannot be read without the backup software/system.
I guess what I'm getting at is - all these external storage and back up systems seem to lead to having the data from my nice Windows PC NTFS drives squirreled away in some other format on some other, often proprietary, box, such that re-accessing my external or backed up data is totally dependent on these external systems.
I understand the back ups - compressing is at the very core of the idea - but I'd be happier if my data existed somewhere in just plain good old fashioned uncompressed Windows ntfs format, so that if everything else went wrong except the drive, I can just pull a drive, plug it into a Windows PC , and there's my data.
I believe this is called a mirror, and I'm reading about mirroring systems right now. You know what I want? A 2-bay NAS that uses NTFS and automatically mirrors the drives. NOT Raid 1 - which I read will not let me pull one drive and use it like a regular drive because of the Raid directory/file system. No, just plain simple real time mirroring. When you write to disk one, write the exact same thing to disk two. Don't convert them into pooled dynamic hoozit's that can't be used separately - just write twice to two independent disks. But I've gone through a bunch of NAS documentation online and none that I can find offer this. It's always Raid 0/1 or jbod.
THEN, I want another box that will back up one of the mirrored NAS drives in a compressed format. Probably incremental backups so that if need be I can go back to a certain date.
Does this exist?