I was running Kernel:
2.4.21-27.0.2.ELsmp

Then I tried to install Kernel 2.6.11

But it will not boot, it says:
VFS: Cannot Open Root Device "Label=/" or unknown-block(0,0) Please append a correct "root=/" boot option
Kernel Panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

I tried using /dev/sda1 instead of Label=/, and I have the ext3 file system built in(not as a module), I also tried increasing the ramdisk size.

How do I get it to boot? Did anyone else get those errors?

Post your grub.conf. I was going to try and answer,but my answer makes too many assumptions.

You mean you replaced '/' with '/dev/sda1' right? You did have the word "Label=" still there right?

-twantrd

I just wanted to reply that I had the same or similiar problem. My fault was not in lilo (or in your case, grub) because the 2.4.22 kernel was booting, but my 2.6.11.7 kernel was not. Even though they were given the SAME information in lilo.conf . Going with this knowledge I determined that it was saying "unknown-block" because it did not recognize the disk (in my case it was a scsi disk) The 2.4.22 kernel had all SCSI device drivers enabled as Modules. But in my 2.6.11.7 kernel config, I had forgotten about my scsi drivers. I needed AHA-7xxx support to run my adaptec 29160n pci controller and therefore be able to use my scsi disks where my root partition was located. So I changed my kernel config to use the scsi drivers and HUZZAH it worked. So in short, be sure you have the correct block device drivers enabled. If you are unsure, enable it as a Module just to be safe. Be sure if you are using scsi disks to enable the corresponding controller. If you are using IDE (majority say yes here) be sure it is enabled. With IDE it should be enabled by default though...

Take a look at my blog
http://www.blogcu.com/ceyhunk/519/