Info-Mac

The backup takes more than 8 hours and still unsuccessful (because I tried to check what's going on, it interrupted the backup process so it failed). It takes more than 8 hours. I try to run it overnight but still...it could take 12 maybe 24 hours ???
Now, at the beginning when I bought my iMac exactly a year ago, I had exactly the same problem, it took more than 12 hours. I still had Apple support, they accessed my computer remotely and changed something and then it was reduced significantly.
Now I need to change those settings again but I didn't ask what they actually did (I should have).
Does anybody know what settings need to be changed to make backup shorter? (4-5 hours is ok, let's say <8 hrs is already better)

Seems that the external HD has a problem. Tried repairing in Disc Utility but failed. Getting all sorts of error messages. I don't know what's going on and what now...

Ok so now the disc is finally repaired
Trying to do time machine backup

ok so somehow it is working now, the time machine completed backup in 2 hours. Can close this topic for now.

"Somehow it's working now" doesn't fill me with a whole lot of confidence. I would like to recommend that you get something more powerful than Disk Utility. It's not a bad utility but there are utilities far more capable of dealing with problems Disk Utility won't touch or even notice. I've had excellent luck with DiskWarrior.