I had a lot of difficulty installing ubuntu 16.04.03 on a 60 GB mSATA SSD card in the APU 3B4. In the end I got fed up with the standard methods using the APU board. So I took the SSD card out, and installed it in a USB case, and installed the same ubuntu on the USB drive using a USB DVD / DVD image running on an Intel NUC. Easy to install that way. No issues. To enable the serial terminal using nano I edited and changed the line in
Other sites suggest other mods are needed but seemed this was enough (after running update-grub). Once ubuntu was installed and modded for the APU I tested the SSD while still in the USB case, which worked fine, so put the SSD back into the APU 3B4 with no difference noted. The right Ethernet port is the one to use for Ubuntu Server under this scenario. Hardware details appended. Hope this helps someone else as it made it much easier...and I like the low power no fan efficiency of the APU boards for what will be a low volume server.
PC Engines APU 3B4 1GHz Quad Core, 4GB RAM, 3 x GigE, 2+2 x USB3, 3 x miniPCI express, 2 x SIM
60GB mSATA SSD module, Phison S11 controller, 15 nm MLC flash, and RoHS compliant. Manufacturer is Data Power / Phison.
Kingshare mSATA SSD External Enclosure Case KS-AMTU02