11.3-BETA1 installed to mirrored SanDisk Cruzer Blade 16GB 2.0, but not for long, S3420GP. 11.2-U6 also failed, same with Samsung BAR Plus 3.1 32GB. The Samsung's installed with 11.2-U4, upgraded to 11.2-U6 ok.
11.3-BETA1 now installed on an old Kingston 8GB SSD, first on a sata port, then switched to a USB port with a StarTech sata to USB adapter.
Thanks to Chris Moore for drawing attention to new recommendations, I will probably switch to ssd's with usb adapters.
Edit: My failed installs seem to be a combination of motherboard and usb drives. Whilst unable to install 11.3-BETA1 on a Supermicro X8SIE the X8SIE will boot 11.3-BETA1 from USB if installed on an X11SSQ first, installed from DVD. A SanDisk Cruzer Blade 2.0 will boot on the X11SSQ but not the X8SIE. A SanDisk Ultra 3.0 and Samsung BAR Plus 3.1 will boot on both. I may stay with USB boot as long I have spares and hardware that can install to them. Newer boards with more ports for boot drives still preferred. Thanks everyone for your help.
Edit: 11.3-BETA1 isn't booting reliably from a USB drive on an X8SIE, S3420GP is ok, stalls at Booting... message.
Method is now an initial install from DVD to a USB attached 8GB ssd sata drive, subsequent installations are from the sata drive; Boot from the sata drive, Attach a mirror, don't Detach the sata, shutdown and unplug it. Boot and Detach the now missing sata, Attach a mirror, Upload a config, Scrub. This takes about 15 minutes with USB 3 hardware.
A FreeNAS vm can also create a bootable mirror, no need to burn a DVD, takes a little over a minute to resilver a Samsung BAR Plus 128GB.