Synology DS920+ question?

[Enquiries]

Hi I currently have a pretty slow DS216J that has been a good little workhorse for me for several years but I am looking to upgrade.
Originally I was looking at the DS720+ but then I started thinking about the possible benefits of the 2 extra bays (and higher ram). I will admit I really like the interface synology uses and although I don't use many of the features, I have considered expanding my use when I get my network equipment setup

The way I use the DS216j Today is I run backup4all on my laptop/desktop and my backups are saved over my network to the drivers (currently setup as raid 1).
then 1x a month I take another HD out of my safe and connect that via a USB External drive reader to the usb port on the NAS and copy over that month's latest files.... that is my fire/Catastrophic copy.

So what I was wondering was with a 4 drive DS920+ system, can i hot swap the 3rd drive to the NAS 1x a month to do those copies (and have it be on a separate storage pool? So Drive 1 and 2 would be Raid 1 on 1 storage pool and then drive 3 would be its own separate (and temporary raid 0 pool? What about drive 4? Can that also be an independent storage pool ( so Drives 1+2 are Raid 1, Drive 3 is Raid 0 for backups and Drive 4 is another raid 0 drive for miscellaneous?

Lastly 1 last question regarding the M2 chipset (since i bought 1 for my new Desktop and it didn't work out the way I hoped)... Synology says its for increasing the cache. Is it really or is adding a M2 chip basically adding another very fast SSD Hard Drive (which is how my desktop wanted to treat it... and not increasing the MB's actual cache.

Thank you in advance.

[ed]

With Synology, you can actually create a RAID1 across 3 or more drives. This would mirror data on all HDD. If you remove this third drive system will still work, but you could not just plug and play it on a computer. You would need to use recovery software to do so. Alternatively, you can create two separate RAIDs / Volumes (or more, one for each drive if need to). Then automate backup to this third drive or to separate RAID. But again it would not be as plug and play.Asustor off Archive option allowing you to remove drive easier, but still need to use their NAS to see the data.Only Qnap and Asustor allow M.2 cache to be used as storage instead. With Synology this only works as a cache. When you upload or read data it first uses this faster media to improve performance.I hope this helps.