FYI, in my case I got the error about 8 invalid sectors and that did last for a while.
Then I saw this message in my syslog:
Nov 13 06:57:35 monster smartd[3359]: Device: /dev/sdf [SAT], No more Currently unreadable (pending) sectors, warning condition reset after 1 email
So I guess it should fix itself and if not the drive should probably be replaced ASAP.
The first message I can find in my syslog happened on Nov 12 at 06:57:35
Nov 12 06:57:35 monster smartd[3359]: Device: /dev/sdf [SAT], 8 Currently unreadable (pending) sectors
So it looks like it repeated the message until 1 day later and checked the drive again and did not find the errors anymore.
I'm still thinking that I'll get a new hard drive. I can get a 12Tb and have 3 drives in my RAID. So if one dies, I still have 2... It's way more trouble to not have a backup.
My existing drive highest temp. is 48°C. So relatively good. Also the drive ran for only 9599 hours (~400 days) so still very young.