anomie's diagnosis is correct; your installation is dropping into the kernel's debugger. Green text on a black background is simply the output of
dmesg(8) done when the kernel is loading & determining what drivers to load. Without knowing more, these errors may be due to two separate issues:
- The kernel is somehow being corrupted when being loaded or executing in memory. To test this assumption, check the memory. Memtest86 is freely available:
http://www.memtest86.com/
- Likewise as you postulated in passing, there may be a problem in the disk partitioning. I would suggest studying Section II of The NetBSD Guide:
http://netbsd.org/docs/guide/en/
If this helps, great. Even if it doesn't, you will have reduced the number of possible causes & have a better idea as to what is & isn't working on your system.