1- I would like to have 4 Bay NAS (name the disk on the drive A, B, C, D) .
I want first to have two NAS hard drives inserted in slots A and B as RAID-1 ( Content A = Content B 100%) however after 1 week , I will remove B and insert a Blank identical size and manufacture NAS drive in B and as I understood it will copy or build the content of A on B . Now the disk I removed I will keep it in another place maybe a fire resistant safe . Then I will do that every week or another week . This way I have a real time redundant and also a backup copy to last week activities incase some force majeure happened
2- Is it possible that the 4-BAY can be split to two RAID 1 example make A with B as RAID 1 and C and D as RAID 1 . That mean the content of A and B is independent from C and D
3- If question 2 above is not possible ( I can not make two RAID 1 in 4-bay drive ) , I am thinking to buy two 2-Bay Drives and attached both of them through some router that will generate two IP addresses
4-Usually my PC is on for a long time months without truing it off . Can I keep NAS drive on as long as the computer is on or it is advisable to just turn it when you want to transfer data from PC internal hard disk to the NAS then Turn it off .
Yes, you could theoretically do that and rebuild first RAID every time you take drive off-site. But this is not really making a copy in a usable way. You would need to use recovery app to actually get data off this travelling drive. Synology has USB copy function. If you connect you external drive, it will automatically create a copy of 1st RAID or any other volume on this external drive. This will use far less system resources since it doent need to recreate a RAID.
You can have several RAID and several volumes on a Synology NAS.
Alternatively you could get two NAS systems. each NAS would have its own IP. You could also automatically synchronise these two NAS for backup purposes.
NAS should be always running. When choosing NAS drives like WD Red, will hibernate drives and NAS when it is inactive. This would save energy.
I hope this helps.