Why does read speed slow towards the end of a failing drive?

[fuzzybabybunny]

I'm backing up a failing 500GB Seagate HDD using ddrescue and in the beginning of the drive the read speed is fast. My disk image is now at 470GB and the read speed has dropped to 8 KB/s...

What could be happening physically on the drive that's causing read speeds to fall as the head moves closer and closer to the outside sectors?

 

[h9826790]

The error correction cost lots of time.

 

[eton975]

What the guy above said. Bad sectors take a long time to error-correct AND the drive must remap it, meaning the read/write head must move to a different location (SLOW).

 

[fuzzybabybunny]

Hmmm... well, I'm creating an image of the disk using ddrescue under Linux. Do far, ddrescue says the number of errors is 0. Not sure if errors = bad sectors though. Does anyone know?

So I guess what a hard drive does is it tries to read a sector, figures out that it's damaged (somehow), recovers the data (somehow), marks the sector as damaged (no longer writing to it in the future), and copies the recovered data to a known good sector?

So in the end I've got a drive with good sectors but lower disk capacity due to all the bad sectors marked as unusable?