Secure Boot active in T61 and/or R61?

[RealBlackStuff]

I'm running into some problems with T61 and R61 laptops.
Some will boot from a Linux Live CD (Mint 8 or 12, Ubuntu 11.10).
Others won't boot from them at all, but will boot from e.g. PC Doctor and other non-Linux CDs.
Is Secure Boot active here, and can that be switched off?
The security chip is disabled in BIOS, and there are no passwords anywhere.
There are no FingerPrintReaders either on these 2 machines.
I have tried both AHCI and Compatibility, tried CD speed High/Normal/Silent, it makes no difference.
I am shtuck!

[miro_gt]

different BIOS versions maybe ?

[RealBlackStuff]

One already has the latest BIOS.
The other had an old IBM/Lenovo BIOS, updated that to the latest BIOS, still no go!
I have an identical other T61 with the same BIOS. There Linux boots fine. They are only 1 month apart in manufacturing date.
Tried Middleton's BIOS on them, still nothing!
I have swapped optical drives, and even tried 2 different USB drives. Still no joy.

Update 12/28/11 3:20pm:
tried a small distro (Puppy Linux) and all is well.
I now suspect that my CD/DVD writer (some cheap SATA ASUS yoke) does not work correctly on nearly full CDs such as Mint 12, even though I did a slow write and had the CD verified.
As these drives are cheap, a new burner will be installed shortly, then I'll try again.