As per my understanding, you have 2 drives already running in RAID 1 configuration however you would like to add 2 additional drives of 2TB each and configure these 2 new HDDs in RAID 1 configuration separately giving you the following configuration at the end:
- RAID 1 on Slot 1, 2: 2 x 1TB
- RAID 1 on Slot 3, 4: 2 x 2TB
Let's break your question into two parts and then answer it.
PART I - Having two RAID levels on a NAS:
Having two RAID levels on a NAS is not possible regardless of the RAID levels i.e. even if you want to have two RAID 1 levels each, that is not possible. You can only have one RAID level on one NAS.
PART II - Adding 2 new HDDs of 2TB each:
Adding two new additional drives of 2TB capacity each in a NAS that is already running with 2 drives of 1 TB each configured in RAID 1 is possible however there is a glitch in your typical scenario. Since referring to Part I above, you can not have two RAID levels on one NAS, the new drives you will be plugging in your NAS will have to be a part of the existing RAID which already has two drives configured of 1TB capacity each. You must remember that whenever there is a mix and match of drives with varying storage capacity, the higher storage space drives teardown to the total capacity of lower storage capacity drives which in this case if 1TB. So when you plug in the 2TB drive, since the existing RAID is using 1TB drives, the capacity of the newer drives will automatically teardown to 1TB each wasting a total of 2TB for you. As a result, rather than having a total of 6TB raw storage with 3TB usable when configured in RAID 1, you will now have 4TB raw storage space with only 2 TB usable space.
So adding 2TB drives in your existing scenario would be of no use and you might want to restrict yourself on buying only 1TB drives.
I hope you get the above clear however in case of discrepancy, you are welcome to post further queries and we'll be glad to help you.
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