stopping disk access

We have an embedded system running 2.4, we need to quickly (within 5
secs) stop all disk access and go into single user mode to run the fail
safe app.

The problem is that killing all processes will take too long before
doing umounts.
But it seems that doing a 'init 1' will quickly suspend all process
without killing them which is fast (true?)
But I can't umount so I'm afraid that the drive would still be
accessed.

Is there someway to suspend drive access, like shutting down the ide
driver?

I'm sure you will probably be asking why, so here's a brief

Our system needs to run certain applications when the hard drive fails.
 We know the hd will fail under certain tempertures, so when we hit
that temperture, we need to stop drive access and run apps from CF.  If
the hd is accessed during this time, the system locks (probably because
of seek errors, or ide driver failing - we don't know).  So we need to
prevent hd access during the failed temp ranges.