Creating a bootable Ubuntu drive
Download Ubuntu 14.04 32-bit via Torrent.
On a Windows computer, burn the Ubuntu ISO image to a USB flash drive with Universal USB Installer.
Lamassu factory image
If your Douro was shipped before January 2017, download the image file for the ACP-1103. Otherwise, download the image file for the ACP-1104.
You can determine which model your tablet is by whether the USB port next to its power button is USB 2.0 (black) or 3.0 (blue). The ACP-1104 has one USB 3.0 port, the ACP-1103's are all USB 2.0.
Copy (as a file) the Lamassu image onto a second USB drive (best formatted as FAT32, and preferably a USB 3.0 drive).
Booting to Ubuntu
Insert the Ubuntu USB into one of the black USB ports atop the Aaeon tablet.
Connect also a keyboard to the tablet.
Press the power button to the tablet, then repeatedly press the DEL key on the keyboard to boot into its BIOS.
Select the Ubuntu USB drive as the machine's boot drive, then save and exit. (screenshots here)
Flashing the drive
When the tablet boots into Ubuntu, connect the second USB drive that contains the Lamassu image to the tablet's blue (USB 3.0) port.
A dialogue box will ask if you want to open it in the file explorer. Click yes, then note the name of the drive shown at top. (example: 7C32-4A05 in the screenshot below.)
Pull up a terminal window with Ctrl Alt T. Run:
Then run the following command to flash the internal drive of the tablet, replacing DRIVENAME with the name noted in the file explorer, and adjusting for the 1103 or 1104 file name.
xzcat '/media/ubuntu/DRIVENAME/1104-20170601.img.xz' | dd of=/dev/sda bs=1M
It will not show an output until it's done, which could take between 15 minutes to an hour, after which the terminal will return to a new line.
Booting to the internal drive
Power off by tapping on the Ubuntu icon and selecting shutdown.
Remove both USB drives, then boot into the BIOS as before and ensure the internal drive is re-enabled and selected as default. Save and exit.
Once booted, initialize and pair to your server as before.