Error 18 comes from a BIOS limitation. The place where the boot files have to be in the hard disk must be before a given size limit.
Whenever you install a new Linux you should reserve a 300 MB size partition for /boot to be put there. This 300 MB size partition should be before the BIOS size limit.
Automatic installation problems
TODO: EXPLAIN WHY UBUNTU DOES NOT TAKE CARE ABOUT /BOOT BEING VERY FAR FROM HARD DISK BEGINNING
Bios Limitation Size Chart
TODO: Copy and paste from Hermanzone and make a chart.