The Story

I recently wiped my CubieTruck (a single board computer, like RaspberryPi), and tried installing the root filesystem on a hard drive instead of the built-in NAND, due to the NAND's tendency to get corrupted.
I used Igor Pečovnik's Image, which is really convenient, as I like it better than the images offered at the official site, and it's obviously much easier than building by myself.
However, I had one serious issue.

The problem

Igor's image is designed to be installed on and booted from an SD card, and includes a script to install the OS to a hard drive, which does 3 things:

  1. Formats and prepares the hard drive
  2. Copies the root FS to the hard drive
  3. Modifies the SD card's /boot directory to load the root FS from the hard drive

Igor, assuming there's only one hard drive, always targets /dev/sda1 as the hard drive. This was an issue for me, as I have two hard drives - an internal one that I installed the OS on, and an external one that I connect every now and then.

The uEnv file (the entry point for CubieTruck's boot sequence) contains something like rootfs=/dev/sda1, and when both my drives were connected, my external hard drive was discovered first, meaning /dev/sda1 was pointing to it, causing the kernel to panic as it couldn't find a proper root fs.
Being used to a standard-built Linux kernel, I modified the uEnv file to look like root=UUID=<PARTITION GUID>, which works around the naming issue.
It didn't work, causing the boot sequence to hang with:

waiting for root device UUID=<Whatever I wrote in uEnv>

The Solution

I tried some tricks, including referring to /dev/disk/by-uuid/, but no luck. Eventually I found this lifesaver, that made me realize that initramfs isn't included in Igor's build, and without it there's no UUID or LABEL mapping, only PARTUUID mapping. Using the value taken from blkid (like rootfs=PARTUUID=<SOME GUID>), the boot went OK.
However, when editing the /etc/fstab file (which also contained /dev/sda1 as the root mountpoint), I noticed that mount PARTUUID=<Something> didn't work. Thanks to this post, I learnt that support for PARTUUID was added to mount, but the Ubuntu version is too old to contain it. This might change, but I worked around this by using good old new UUID in /etc/fstab.

Now all is well.

TL;DR

Use PARTUUID in uEnv, found via blkid.
With /etc/fstab, test with mount before using. If it fails, work around with UUID.