I have tried to change the clock rate of a motherboard on two ocasions. Both times before any expected stability problems it caused trouble with the HDD.
First I overclocked an Intel 440LX from 100 to 133 MHz and the HDD was corrupted upon next boot, back at 100 megs. Now I tried to downclock a superhot Pentium 4 by going 133 -> 100. The connected HDD (another one, sata) was undetectable by BIOS.
Both systems still work properly at original megahertz.
Is this normal?