I am doing a linux course. On a Windows 7 PC I have installed Virtualbox. Inside Virtualbox I have CentOS 6.7 and Ubuntu.
Now is the topic rescue cd. So I have downloaded Knoppix 7.6.1. I can boot the pc with the DVD and get what I guess is the grafical UI of knoppix.

However, when I use Virtualbox and try to start the CentOS virtual machine it starts to boot from Knoppix OK but stops with:
Konnte Disk nicht nach /mnt-system mounten. Starte debugging shell:
sh: can't access tty; job control turned off

Can somebody tell me if this is supposed to work or am I trying the impossible?

Thank you and regards,
Mark