I'm reminded of an old joke. You've connected with a backup fanatic here; I could tell you stories (by PM if you want to hear them).
A few points. What is this "snapshot" of which you speak? The word refers to a kind of backup and may also be the name of a package that you are using. I have just tried Acronis True Image, and it will back up data from a network drive. It's not that expensive, and lots of people that I know use it.
Plus, rather than take the time to make this into a coherent whole, here are some disconnected sections separated by the following dot:
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Second, a "clone" is an exact copy of one drive to another. You could disconnect the first drive and plug in the second and the system would run fine, even if this was the system drive. No special software is needed to read the copy.
An "image" is a large file (or set of files) created by the backup software that needs the backup software to read it. Without the backup software, it looks like one huge useless file. The backup software can unpack the image onto a new disk, creating a clone on demand.
For example, Acronis True Image and Norton Ghost 8.0, which I use (that's an old, old version) can take an input disk and produce either an exact clone or an image file. I personally use image files because they are compressed, they can be made faster (less data to write), and I never want to have more than one bootable disk drive on my machine.
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There is no "ideal setup," so don't worry about getting it wrong. Various approaches have various strengths and weaknesses, and even having off-site backup copies in another state is vulnerable to unlikely events such as a blackout of the entire East Coast, to use a real example.
A decent setup will protect you against one or a combination of a few unlikely events.
As for backing up your NAS to another RAID0 NAS, I would like to discourage you for two reasons. First, RAID0 is not a reliable way to store data, and I would never use it for a backup. A single drive, or even a large backup with 3 GB on one drive and 3GB on another, is more dependable. Second, having your backup on the same network as your data opens you to a vulnerability that you can protect against: Malware destroying your data. Backups, or at least copies thereof, should be offline.
It makes sense to do realtime incremental backups (continuous backups) to another storage device on the NAS, but not image backups. There's a point below on the difference if you're not familiar with it.
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The usual backup process is to make a full image backup of all files, followed by multiple "incremental" backups that only copy files created or changed since the last full or incremental backup. That way, you can backup daily but not need to copy the huge contents of your storage system over and over again.
You should have media for at least two full backup cycles. If you only have media for one full backup, imagine the following: You start a backup, putting your only copy in the drive to write on. The system fails in the middle of the process. You now have no backup at all. So, have at least two complete sets of media to back up to.
Many people have made do with a single set for decades and never lost data, but I am being very careful here.
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I can't help you with the main question: how to perform the backup. I'm not familiar with this "snapshot" that you are using, nor with the NAS backup software.
If I were you, I would read the NAS manual and find out how to do a backup to a drive on the USB connection. Especially, find out if the backup can be "spanned" across more than one disk when your data grows to more than the size of a disk, and find out if the backup disks can be read easily or require a version of the software to read. In the latter case, burn a bootable CD with that software, so that you will be able to read the backups even if the NAS and your PC are toasted.
If you want to back up your data via network, there is one very very simple way that will work. Mount both drives to one PC and do a copy. Boom, it's all backed up. Not elegant, but it will work.
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Network Backup need not be your goal unless you are going to run continuous backup. After all, unless the backup software is native to both of your NAS units, the backup will have to run from a PC and two copies of the data (pc read, pc write) will travel over the network. This is slow.
For continuous backup, what you should do is still do a full backup that is taken off-line. Then you can run your continuous backup software to copy files that are change on the storage NAS to the backup NAS. If the storage NAS fails, the backup one will be there with all of the files.
But backup to network storage is like RAID1. It increases the reliability of your system, allowing it to survive certain failures. It is not, in my opinion, an actual backup. It's closer to being an actual backup than RAID1 is, since the copy is further from the original, but I'm not satisfied with my backups until the drive is removed from my PC and safe in my antistatic storage box. And if I were serious about it, I'd keep the completed backups at a friend's house.
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I've given you a disorganized lot to chew on, because it's a big question and the answer depends. I'll try a short answer, giving just my personal preferences.
If the NAS local backup software permits, back up the NAS to directly-attached drives, spanning two drives as your data grows. Do full backups periodically (monthly?), and incrementals as often as the most data you are willing to lose (a day's worth, a week's worth, ...). Have two sets of backup media and alternate them.
Do the same thing for data in your computer. Oddly, I never do incrementals of my OS drive, only image backups. If I lose the OS drive, or it gets corrupted, I restore the last backup and I'm up and running. I have to re-do Microsoft Update and any software I installed since the last image, but that's cheaper than rebuilding the OS.
Let me know if you have specific questions about one of the paths I've laid out here, or something else.