The continuing conclusion is not to use seagate drives under 4TB there notoriously unreliable. The sweet spot actually seems to be 4TB drives where most manufactures do quite qwell, but HGST beat everyone.
Anything I care about that is not easily recovered - throw it on a USB HDD. You can copy to multiple drives. As with any backup, you should also ensure that you can recover from the backup (if it was a simple copy, perhaps copy it back to your main rig and try to open the resulting copy).
You can use online storage depending on how comfortable you are with that. For the USB HDD, you can rotate weekly and keep it at parents/inlaws/a friend. Physically having the backup elsewhere is the best thing, but it all depends on importance of the info, trust and cost :)
You could also run a hash check. That way you're 100% sure that the backup is exactly the same as the original. One bit flip in an image may go unnoticed when you're quickly checking a backup of hundreds of files. A hash check will show that the files' checksums don't match.
Linux has MD5 out of the box. If you put your backup in a linux box you could automate the integrity check and send yourself an email if something has a problem.
Okay. I ended up with 2x 5tb discs and I'l get them next Week. One of them I will put in my computer and the other I will use as an external backup drive that I will store in my parents house(about 100m away). Is there some program that can scan my drive and backup data that is'nt already on the drive automaticly when I connect the drive or start the program?
oh well, I'm actually using raid 1 via mdadm for the fault tolerance. so many drives have failed me that I really don't want the whole PC to stop working if a disk is failing. software raid ain't that bad if you know what to use it for but I'm gonna use btrfs raid when I'm upgrading my disks again.