Hi, i have had a PI3 with Dietpi OS and Nextcloud installed. I have created RAID1 with mdadm. Worked well though sometimes a bit slow. I did this arrangement to keep the system alive in case of trouble until i can fixit.
However it just failed and i was not able to get the data out of the array. So all gone. Now i rebuild the system and i would like to ask which is more preferable setup: with RAID1 and backup or without it (one disk) and then backup. It is a personal cloud anyway, so do i really need RAID1? thank you
RAID (no matter which) is not a subtitution for a reliable backup strategy 
I also use nextcloud, but I do not have so important date on it, I use it more for sharing. My “backup” is one of the clients, it automatically syncs everything to it.
I think also you get no speed benefits with RAID on a RPI3, because the bus is shared with network and the USB ports. And now you have a RAID 1 with two drives, which may bottlenecks it even more 
If you want a somewhat more robust RAID, you could use RAID 5, but you need at least 3 drives for it.
And in parallel a backup strategy. A software raid has always a potential to fail.
Thank you so much. I have only one more quesiton:
in case the array fails what is the way to take the function disk data out.
depnds also on the type of RAID, example for RAID 1:
How to recover from a broken RAID array with MDADM - Knowledgebase - Woktron Ltd. Company
With Raid 5/6 you replace the faulty drive and rebuild with the help of your parity data.
Thank you so much!