At least a RAID does not need software to compare backups.
The recommendation is to still backup data on a NAS even with RAID configuration.
With RAID 1 (Disk Mirroring) one drive can die, but you only get 1/2 the capacity of the smaller disk if they are unequal. With Raid 5 (which requires a minimum of 3 disks), you get greater storage capacity - if all disks were equal, 3 disks would be 2/3, 4 disks would be 3/4, 5 disks would be 4/5 etc. of total capacity. Again, one disk can die and be replaced - the data is spread across all drives and a parity is kept, so that when a disk dies, the parity and the remaining data is used to recalculate 'the' data on the missing disk. It works but it can take hours for the NAS to recalculate and reconstruct the data on a large disk. RAID 6 is a similar to 5, but with requires a minimum of four disks and there is a greater hit on overall capacity. With RAID 6 there is greater redundancy and 2 disks can die simultaneously in the pool. Once the 2 disks are replaced the data on the disks can be reconstructed from the data on the remaining disk, and you are once more good to go.
I suppose the problem is that is possible for something catastrophic to happen and for you two loose more than 1 disk (or 2 disks with RAID 6) simultaneously - a bit like what happened in my 2 drive HDD docking station, hence the reason for backup. Perhaps a large power surge or burn out of a component in the NAS etc, could corrupt all HDDs in the pool.
The rule of 3 is that you have two backups of your data and keep the backups in different locations (perhaps one on disk and one in the Cloud), but this is pretty tough to do, particularly for an amateur enthusiast, having to pay for storage. I (now) try to keep storage and backup apart to be cautious. This is why an external drive can be useful. I have started to use the rule of three for stuff that I consider really important, with a copy on the Cloud or for stuff I will never edit again Blue-Ray M-Disk.
Sometimes the disks can ONLY be used in THAT particular NAS
Yes. For example, Synology have Hybrid RAID, this is kind of like a mix and match of RAID configurations to get the most capacity out of your drives if they are of different capacities. With standard RAID configurations the capacity is governed by the size of the smallest disk - say you had 4 3TB in RAID 5 (9TB of overall capacity) and you upgraded 2 drives to 6TB, you would see no change in capacity, as the data is spread across all the disks. With Hybrid RAID the lost capacity on the two disks would be used as an additional RAID 1 configuration, so you would get an extra 3TB (total 12TB).
The problem is this is Synology configuration (not an industry standard), so if the NAS drive dies, you would need to by a new Synology NAS - you couldn't use the disks in a QNAP NAS or another brand. I understand with standard RAID configurations the disks should work in any NAS (with the same or greater number of bays)
I understand software can be used to copy data for a dedicated NAS drive to a Windows one. It is not free, but in this "business" better to do it by the book.
Yes, I use Arconis True Image (now Arconis Cyber protect home office) - other software is available. Arconis currently costs £34.99 annually (they quite often have sales though). The software does disk cloning, backups (including incremental backups) etc. You can create multiple backup sets, with each set backing up specific files, folders, a group of folders or a drive (including files or folders on a NAS). You can also select different targets for each backup set, so can keep different backups on different hard disks. Each backup set can run automatically at a set interval/time, or you can trigger the backup to run manually. The only issue is that incremental backups do take a long time to complete - not sure why this is?
I think I had got the backup software right - some time ago the main disk on my PC died, so I invested in the software and a new SDD at that time to make recovery of the system and data easy. As I say my main mistake was keeping the original and the backup in the same location, and I think you have teh same risk if you use a NAS for both. Better to separate source and backup physically.