Time Machine Error

Hi
I am a newbie to Mac and of course OSx 10.5.5
On This Imac i set up an external drive and said yes to the question did i want it to be used for Backups.
There is still a lot of capacity spare on the backup drive, but i regularly get a message to say that TM backup has failed.
As a newbie, i have no idea how to treat this, is this just a temporary glitch? will the next attempt be ok? and so on, or is there some action i need to take to help remedy these failures.
Can someone help plz

TIA Len

sometimes happens to me aswell. dont have a solution for that though. i just unplug the drive and plug back in

Happens to me sometimes as well, I just say okay and then run a manual backup a few minutes later and it works fine.