I just tried to boot Knoppix on a pair of new Dell Optiplex 390 systems. I get:

Could not mount disk to /mnt-system. Starting debugging Shell...
?: can't access tty; job control turned off

(The ? falls of the edge of my cheap old LCD).

I got this error with Knoppix 6.4.4, so I burned a new 6.7.1 disc and got the same error. I have tried the following cheat codes:

knoppix vga=0
knoppix acpi=off pnpbios=off noapic noapm
knoppix vga=0 debug -b 3
failsafe debug -b 3

The system has a Pentium G630, on-processor video (HD Graphics 200), Intel H61 Express chipset, 4GB RAM, SATA hard drive and DVD drive.