Trying to create a bootdisk displayed the following message: Cannot find root partition in /etc/fstab I think the problem comes from the fact that at the line 119 in the /sbin/mkbootdisk script the previously found value for rootdev is changed to the empty string. Simply moving the lines 132-135 *before* line 110 has solved the problem.
I have not looked at the mkbootdisk code, but changing my /etc/fstab from using LABEL to the actual device name seems to have made mkbootdisk happy. I presume not supporting LABEL is a bug.
It works with LABEL= here.