How old is old ?
What DVD reading software are you using ?
A DVD will take longer to read data than a CD as there is more to read.
Is the hard drive nearly full ?
Has the hard drive been defragged ?
There are many reasons, but if you are selling it and the new owner knows of the problem, no problem.
You know of course that any software on the hard drive, and the CD's/DVD's that load it should go to the new owner.