For RAID 1, you CAN access the data when one of the driver die. You can crate and simulate this process.
1) Plugin a USB stick (e.g. 16G)
2) Use disk utility to crate a partition a 16G partition on you HDD.
3) RAID them together in disk utility (RAID 1 of course)
4) Now you have a 16G RAID 1 partition, just copy some files to that partition.
5) eject the USB stick
6) Now, your RAID 1 partition is broken, but the data on your HDD still there and accessible.
Anyway, RAID 1 is not a real backup solution, it just keep your system alive. If you accidentally delete an important file, the same file will also be deleted on your mirror HDD and can't be recover.
For system backup, I prefer to use Carbon Copy Cloner (CCC), just schedule it to clone your boot disk to the standby disk (let's say once per day). If you accidentally break your OS, you can still boot your computer with the backup disk, and clone it back to the primary disk. the CCC can also serve like the time machine. However, I personally prefer to use time machine to backup / recover a particular "version" of files.