I have installed two identical hard drives with software RAID1 on all of the partitions with two exceptions. See https://forums.mageia.org/en/viewtopic.php?f=8&t=7975 The exceptions are the SWAP partitions and the MBR on both drives.
The boot loader was installed in the MBR on both drives using the following process:
1. Launch the "Mageia Control Center", select "Boot', select "Set up boot system". Then select one of the hard drives as the "Boot Device" and the other options as desired, and select "Next". Set the options as desired, and select "Finish".
2. Repeat step 1, using the same options, except select the other hard drive.
Later Problems Happened:
I was installing software upgrades that included a new kernel. After the upgrade was done I tried to shutdown the system to reboot it. The shutdown seemed to take too long, I became impatient, and held down the the power button for several seconds. MISTAKE. Then when I powered up the system I got "L 99 99 99 99 99 99 ... 99 99 99" for several lines on an otherwise blank screen.
I cycled power on the system, and typed a key to go into a menu to select a specific drive to boot from. I selected the second hard drive in the list. (Booting from the first drive was giving me the error message.) The system gave me the expected boot menu, with the list of kernels, and started up cleanly. Then I repeated the process of installing the boot loader in the MBR on both drives.
Now the error message does not occur and the first drive boots up cleanly.