What is happening when I read a corrupt optical media with dd and it's really slow?

I have a DVD that I can't rip or copy using regular commands. If I try to copy it it will first copy at normal speed, but then slow down and take a very long while to do seemingly nothing. Right now I'm trying to extract whatever data there is using dd to make a .iso-image of it. I use the following command: dd if=/dev/sr0 of=image.iso bs=2048 conv=noerror,notrunc iflag=nonblock noerror: continue after read errors notrunc: do not truncate the output file nonblock: use non-blocking I/O Now the extraction has come to the difficult part and I get messages like the one below all the time: dd: error reading ‘/dev/sr0’: Input/output error 486221+0 records in 486221+0 records out 995780608 bytes (996 MB) copied, 76031,9 s, 13,1 kB/s It ticks up a few bytes for each minute. As you can see this has been going on for a while now ... **So my question is: What is my computer doing now?** The drive slowed down, but is still able to extract some data (the byte counts increases)? Does the lower speed make it easier to read the data, or is the drive just trying to read the same data over and over until it gets some of it? **What is happening?** :)