HP check for recovery disk validity

I got my HP DV6500t notebook in august 2007. I wanted to get rid of the recovery partition so I made recovery DVDs and then repartitioned the whole system as per my needs. I did once use those DVDs a few months later to recover my system and they worked fine. A few days ago when I again tried to recover the system using the disks I could not do so. I got the infamous : "This PC is not supported by the System Recovery Disks. You will not be able to continue to recover this system from these discs" error. Which made me wonder why did this happen. Does anyone know what HP checks for when it uses the recovery CD/DVD to determine if that system is supported to be recoverd by the particular CD/DVD.



Here is what changed during the period since I did the recovery the first time and the attepted the second time:

1) I updated the bios (sucessfully) about 2 months ago.

2) Back in August 2008 I sent my notebook back to HP for repair. The trouble was that it was dead and won't boot up. HP told me that there was a motherboard issue. They did not tell me if the replaced the motherboard or did some repair to it.



To me one of these two could be a potential reason for DVD not recognising the system as same as before. Does anyone know for sure what HP recovery DVDs check for to determine if a system is supported or not?



Point to be noted the DVD was not corrupted/scratched. I know for sure because I had made ISO images to back up my recovery disks and therefore also tried to make fresh DVDs to recover the system but failed.



I have already got replacement recovery DVD from HP and recovered my system back but am still curious why this happened. I must know this to I make sure that DVDs I made this time won't let me down next time I appempt a recovery



Thanks