Aside from a backup; one of the non parity raid levels, ie:
btrfs raid1 (2 copies on 2 different devices) min drive count 2.
or
btrfs raid10 (raid1 with striping for performance) min drive count 4.
Both levels effectively have single disk failure robustness:
see: Redundancy profiles in the Pools section of the docs.
Note that currently btrfs is only 2 way in non parity raid levels so irrespective of how many disks you have only 2 copies of the data / metadata will be made, but they have to be on any 2 different devices (for raid1/10). With btrfs the raid levels are at the block/chunk level not the device level, but are device aware in order to duplicate appropriately given the btrfs raid level used.
Hope that helps.