Just had an issue where I could not mount my USB drive at boot time I was getting an error along the lines of UUID=blah not found...I previously had performed the standard "sudo blkid" and received the following output:
[corte@ashbox ~]$ sudo blkid
/dev/sda1: LABEL="Boot" UUID="3a31d197-0436-4b87-9f06-b03080a3772f" TYPE="ext2"
/dev/sda2: UUID="ea73d633-0f71-43a3-9101-6461f19b1222" TYPE="swap"
/dev/sda3: LABEL="Root" UUID="63b1459a-3ee6-4d7a-9afa-9ad40db800bd" TYPE="ext4"
/dev/sda4: LABEL="Home" UUID="00e59ee8-019b-4ad2-9171-e8501a1022c3" TYPE="ext4"
/dev/sdb1: LABEL="Elements" UUID="7da36762-56c3-471d-bfa1-5b4895bfa0d7" TYPE="xfs"
Noting the UUID of my external USB drive (Elements), I added the following line into my fstab file:
UUID=7da36762-56c3-471d-bfa1-5b4895bfa0d7 /media/Elements auto defaults 0 0
Everything went dandy with a "mount -a" yet at the next reboot, I received an error stating that the device with the UUID 7da..could not be found. A bit of research lead me to the solution...
Including "usb" before "filesystems" in the hooks section of your /etc/mkinitcpio.conf file:
HOOKS="base udev autodetect pata scsi sata usb filesystems v86d"
Regenerate your mkinitcpio file with:
sudo mkinitcpio -g kernel26.img
Then be sure to make a backup of your old /boot/kernel26.img
sudo mv /boot/kernel26.img /boot/kernel26.img.bak
Finally, copy your new kernel26.img to /boot.
cp kernel26.img /boot/kernel26.img
Now reboot and your external drive should be mounted automatically with no issues.