Read up on Knoppix "cheat codes". memtest (not memtest86)is the parameter to use to run memtest86. Don't use the word knoppix like you do with most other cheat codes (the first parameter is the kernel or program that gets run).

Unfortunately, a high speed burn is usually the default option, so you don't have had to select it. Selecting a low speed burn is even becoming difficult or impossible with some software that wants to look fast. I've lost count of how many times I've seen Knoppix problems that the user swears can't be because of a bad burn but none the less go away when they finally do a low speed burn. There is a lot of redundant error correction information on a CD or DVD. You want that error correction to help fight bit-rot over the years, not to hide marginal results from a high speed burn. I didn't want to believe that it was important to burn at low speed at first either, but years of experience have convinced me it is.