I'm attempting to upgrade my current backup system where external HDDs are manually connected to my system & a backup is controlled by Beyond Compare.

I have been considering 2 NAS/DAS drives, one onsite, the other offsite. I have 4x1TB WD Green & 1x3TB WD Green HDDs that I would like to continue to use until they fail. I favour a Synology DS416 as the onsite NAS, furnished with new 2x4TB WD Red HDDs. The offsite would need to be 5-bay & compatible with the older HDDs. If USB3, then it would need an ethernet adapter to accommodate long cabling.

The plan would be to backup to the Synology DS416 from my PC & sync the Synology to the offsite storage. I'm not convinced either would need RAID as they are mirrors of each other & any failed drives could be repaired from the copy. I had been told that Greens will not RAID without corruption. Can anyone see failings (apart from the cost) in this arrangement, advise on compatible devices or any other constructive observations?