Data wiping PERC array

Hi all

I had a really frustrating conversation today with a chap who insists that you can't wipe data from a RAID array without doing a low level format.  I use Active Killdisk to run a single pass/write zeros method and he says this is not enough, that the controller will cache some of the writes and leave the data on the disks intact.  I know the PERC4/Di is good but just how much of a four 38GB disk set does he think can be cached?  And as I remember, the cache is flushed on system shutdown?

The array was RAID 0+1 so I reconfigured it as RAID 0, ran the single pass, reconfigured it as RAID 5 and reinstalled the OS.  It made more sense than initializing four disks individually - at least I could set it running and walk away.

He also says that even with a successful write pass, it is still possible to read the previous data.  I didn't think the drive electronics could do that - that you would have to remove the platters and analyse the disk surface with something more advanced.  He seems to think even that is a possibility.

The last bit was my favourite - apparently a RAID controller won't write a zero if there is one already there.  I'm not clear why a pre-existing zero would hold more information than a new one?

Please tell me, is there anything in any of this?  Am I just missing the point and he's saying something valid here?  Data security is a very touchy subject these days, particularly in the goverment and banking sectors, which applies in this case.

Many thanks in advance!