[Xbianfan]

Yes of course, that's what I did:

I followed almost this guide http://wiki.xbian.org/doku.php/installation_usb (Older versions) and changed root= to the external drive in boot.scr.txt.

The exact steps are:
1. Unmount external drive and make filesystem:
sudo umount /dev/sda1
sudo mkfs.btrfs -L xbian-usb /dev/sda1

2. Mount it and copy files from sd-card
sudo mount /dev/sda1 /mnt
sudo rsync -aAX /* /mnt/ --exclude={/dev/*,/proc/*,/sys/*,/tmp/*,/run/*,/mnt/*,/media/*,/lost+found,/home/*/.gvfs} --progress

3. Edit boot.scr.txt
sudo nano /boot/boot.scr.txt
change root=/dev/mmcblk0p2 to root=/dev/sda1 (or what your partition on the external drive is)
remove subvol=root/@,

and compile boot.scr with:
cd /boot; sudo ./mks

sudo reboot

That's all!


UPDATE: After that the BTRFS-volume "Root" is missing, which is producing an error then using apt-get.
To fix this, create it with:
sudo -i
cd /
btrfs sub create ROOT
btrfs sub snap / ROOT/@

and then re add the previous deleted text at the same position:
sudo nano /boot/boot.scr.txt
subvol=ROOT/@,
and compile it again with: cd /boot; sudo ./mks

(It's important that ROOT is written the same way as created, in this case in capital letters!)
For more informations see: https://github.com/xbianonpi/xbian/issues/556

__________________________
I use an eSata -> Sata cabel with a usb-connection only for the 5V. The USB-cable is connected with a powered usb-hub, because it did not work with the usb-cable connected directly to the cubox-i.

UPDATE: Updated the wiki-page for usb-installation: http://wiki.xbian.org/doku.php/installation_usb
Hope it's all right I'm not an expert, it's still more trial and error.