Mirroring worked for me, everything I needed was included, but I now work without mirroring and use the entire memory. I didn't have any charges. When mirroring, only half of the memory is available. External hard drives for additional storage are also no problem.

I use my NAS storage with RAID1 for my photos, so data is stored on two drives each.
No app is required for RAID1, but a selection of apps is displayed, e.g. for server operation, which can be installed and some of which are subject to a charge. There are certain tools for checking data carriers and the system under Settings, which should determine whether the mirroring is working properly or whether errors are being detected. At least that was the case with my Buffalo NAS devices. Takes a correspondingly long time for a complete check.

Good afternoon,
No, I haven't noticed anything about that yet. It seems to me that mirroring should work just like that, with the appropriate settings. But I'll have to check whether the second HD is really being filled....

Raid 1 is correct. As I wrote my backup software myself, I don't have any costs. The only problem I still have is with the automatic backup of several mobiles with access to the cloud. Unfortunately, I don't have time to solve the problem at the moment.