Change default filesystem root

Question

I have an external hard drive that I want to set as the filesystem root but boot from an SD card. How would I do this?

I've copied the filesystem to the partition, and tried updating config.txt with root=UUID={UUID} and cmdline.txt with root=UUID={UUID}. However, it still fails to start up and never completes the boot process, usually ending with kernel panic.

What am I missing?

cmdline.txt:

dwc_otg.lpm_enable=0 console=tty1 root=UUID=96fd5e33-3220-37e9-af31-b87ea8d03dda rootfstype=hfsplus elevator=deadline rootwait rootdelay=5

config.txt (different than default)

root=UUID=96fd5e33-3220-37e9-af31-b87ea8d03dda
initramfs initrd.sda

lsusb output:

Bus 002 Device 002: ID 1058:259c Western Digital Technologies, Inc. 
Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 001 Device 004: ID 04d9:0006 Holtek Semiconductor, Inc. 
Bus 001 Device 003: ID 05e3:0610 Genesys Logic, Inc. 4-port hub
Bus 001 Device 002: ID 2109:3431 VIA Labs, Inc. Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

lsusb -t output:

/:  Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/4p, 5000M
    |__ Port 2: Dev 2, If 0, Class=Mass Storage, Driver=usb-storage, 5000M
/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/1p, 480M
    |__ Port 1: Dev 2, If 0, Class=Hub, Driver=hub/4p, 480M
        |__ Port 3: Dev 3, If 0, Class=Hub, Driver=hub/4p, 480M
            |__ Port 1: Dev 4, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
            |__ Port 1: Dev 4, If 1, Class=Human Interface Device, Driver=usbhid, 1.5M

Answer 1

I solved my problem by doing a couple of things. First of all, my drive had been formatted as an HFS+ drive, so I suspect that that played a factor in preventing the RPi from booting correctly, and secondly I was using the UUID instead of the PARTUUID.

What I did to make it work:

1. Repartitioned the drive to take full advantage of the space
2. Formatted the drive using mkfs.ext4 to create a new ext4 filesystem
3. Copied the data from the SD card to the new drive with rsync -ax
4. Retrieved the PARTUUID via lsblk -o NAME,PARTUUID
5. Created an initramfs with mkinitramfs -o initrd.sda
6. Added initramfs initrd.sda to config.txt
7. Replaced the root= lines in config.txt and cmdline.txt with root=PARTUUID=partuuid
8. Added rw to cmdline.txt to mount filesystem as read-write and not read-only

Changes to boot files: config.txt:

root=PARTUUID=partuuid
initramfs initrd.sda

cmdline.txt:

console=serial0,115200 console=tty1 root=PARTUUID=partuuid rootfstype=ext4 elevator=deadline fsck.repair=yes rw rootwait