PROBLEM: rootfs problem on cd-rom

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I have tried to create a standard live cd to scan for viruses with a 
popular linux antivirus software. At first I have tried with the 
2.4.18-bf2.4 precomplied kernel, and it was fine. I have tried with 
2.4.20-bf2.4, but it was wrong. I have downloaded the 2.4.18,2.4.20 
and 
2.4.21 kernel sources, and tried to complile it to boot the kernel and 
the 
rootfs from cdrom. The 2.4.18 kernel, with the standard config was 
fine, 
it was booted. The 2.4.20 and 2.4.21 kernel with the same config has 
fail.

The 2.4.20, and 2.4.21 said: Kernel Panic, VFS: Unable to find root fs 
on 
"hdb".

Thx.: 

Krisztian Fekete