A quick inspection of the code confirms that dma is on in 5.1.1
Behavior shows the same for 5.0.1 - the "nodma" cheatcode solves some boot failures.

In some of the cases of "cannot find knoppix file system" where none of the cheatcodes helped, we recommended a USB cdrom drive - we were able to boot in most of the systems that failed that way. We weren't able to open them up and inspect or change anything - strictly remote support.

Newer versions of the same model Dells that failed to boot 6 months ago now work fine - some change in hardware or BIOS probably.

Maybe check and see if there is a BIOS upgrade available for your system?