Yesterday, while trying to fiddle around with installing and placing my second Hard Disk Drive (HDD), I kept pulling the IDE cables (those flat cables that run from your Main Board to the HDD) out from their slots by pulling the cable itself.
BIG mistake! After just 3 pulls, the cable was trashed and the first symptom is that your BIOS doesn't find any HDDs on your computer at all!!!
So, if you must remove the cables from the slots for whatever reasons, DON'T pull the cable itself, unless you have a spare or you like spending money unnecessarily.
As difficult as it is, remove the cables by only handing the heads of the connectors and do it gently; and if you're still struggling, don't forget to dry your sweaty hands often!