I'm not a mandrake expert, but I'd suspect that the best way to do this would be to boot the drive as is, without the upgrade, in single user mode.
I'd then edit the /etc/fstab file and comment out references to the 3 old drives.
Then I'd do the upgrade. The installation process is probably checking fstab to get the old drive information.
I hope that the missing drives only have data and no operating system directories like /var or /usr. It could be tricky if they do, because the upgrade could fail if key directories cannot be found.