I made the same mistake second time. Again, I loaded dvd2 after dismounting dvd1, without knowing the DRIVE Letter of dvd1. Due to the smartness of the great Windows system, dvd2 was assigned another DRIVE letter, making the Matlab installation impossible.
Tip: 1) load DVD2 ; 2) go to the DISK Management tool of your Windows; 3) rename the Drive Letter, D 4) try Matlab installation again, if it fails again, rename Drive Letter to E, 5) finally you will be lucky enough to find the right one.
This allows you continue, otherwise you need to CANCEL the installation and Try again.