Ubuntu MATE on raspberry pi 3 b+

Yes you can install Mate 16.04 on the 3B+ and you can usb boot it as well. Here is how I did it. You will need to copy certain files from a raspbian stretch installation initially that will be copied to the Mate sd card after it is installed as explained below.

  1. Download and burn the Ubuntu Mate 16.04.2 image to a micro sd card using etcher.

  2. In order to avoid errors when trying to update Ubuntu Mate, use gparted on a Pi with rasbpian installed or a linux computer or gparted live bootable image on a windows computer to increase the size of the Mate sd card fat16 PI_BOOT partition to at least 128mb or larger. Both partitions on the sd card must be unmounted before the resize/move operations can be completed on both the PI_BOOT and PI_ROOT partitions. Resize the ext4 PI_ROOT partition first leaving room preceding it to be used for increasing the size of the PI_BOOT partition and apply the change. Then resize the fat16 PI_BOOT partition leaving no extra space before or after and apply the change.

  3. Copy the following files from an updated raspbian stretch installation to the Ubuntu Mate 16.04 sd card (note that sudo/root permissions are required for some of the copy operations below:

For booting - copy bootcode.bin, fixup.dat, start.elf, bcm2710-rpi-3-b-plus.dtb and kernel7.img to the Mate PI_BOOT partition.

For keyboard/mouse - copy /lib/modules/4.9.80-v7+ to the Mate PI_ROOT partition /lib/modules/ folder

For network/wi-fi - copy the contents of /lib/firmware/brcm/ to the Mate PI_ROOT partition /lib/firmware/brcm/ folder

  1. Booting Mate from usb requires the same sd card as configured above in addition to installing the Mate image on the usb device.

If you want to boot from USB edit these files in PI_BOOT partition on the Mate sd card:
edit cmdline.txt, change root=/dev/mmcblk0p2 to root=/dev/sda2 (PI_BOOT partition)

On PI_ROOT partition on usb drive edit etc/fstab, change/dev/mmcblk0p2 to sda2 and /dev/mmcblk0p1 to sda1 (PI_ROOT partition)