bootable drives suddenly wont work?

I have primary start up disks/volumes in Lion & Snow Leopard.

They have been bootably cloned to other internal and external drives for back up.

Suddenly these back ups do not work. They appear in system preferences>start up disk,

but they never engage. These troubling start up disks/volumes also suddenly dont appear during

startup>option key either. These used to always work. All these disks/volumes appear in disk utility.

They all appear on the desk top. When I use time machine to back

up to another one of these volumes, it says there are no disks available, even tho I used it 2 hours ago.

Time Machine says make sure the disk in question is not on the "skip" list. It is on the skip

list, but it it greyed out, so i can neither delete it or add it.

This complaint is valid for both my Snow Leopard & Lion OS disks/volumes.

Can anyone suggest a troubleshooitng strategy for this kind of thing?

Thanks !