After 11 years of use one of the backup HDDs has finally failed. I can hear the disk still spinning, do it must be the controller or head mechanism.
I don't know about you, but I think 11 years for something pretty much in constant use, spun down and back up daily is not.
11 of more years is the claimed/expected MTBF.
As it's a backup, i'm not too troubled, because, I'm sure you would guess, I keep more than one backup.