http://forums.devshed.com/linux-help-33/initrd-usb-boot-332584.html

> I run linux off my 40Gb external usb hd.
>
> Usually this appears as /dev/sda, but that changes depending on what else is
> plugged in. i.e. sometimes it might be sdb or sdc etc.
>
> Naturally it is important for linux to know where the / filesystem is so i
> have two entries in grub -> one with root=/dev/sda1 and another with
> root=/dev/sdb1.
>
> Also i have to change the entry for root in /etc/fstab

You can do this with a filesystem label. Make sure your initrd image contains the /sbin/findfs binary (and the shared libraries it needs), then you can set a label for the filesystem ("My-Linux-/" perhaps, and assuming you are using Ext2/Ext3):

# e2label /dev/sda1 My-Linux-/

Then, in the grub config you can use "root=LABEL=My-Linux-/" and have the initrd use findfs to automagically mount the proper root filesystem using this label (using pivot_root and LABEL=My-Linux-/ as the partition identifier and not /dev/sdsomething).

documented on: March 9th, 2006, codergeek42