Experimenting with a quad R-Pi 2 to try booting a system from a USB pen drive. I used a Philips USB 3 64GB pen drive.
I was guided by but did not follow exactly the excellent Sam Hobbs article at http://tinyurl.com/kfllzxr
Used dd to copy partition 2 from a working Ubuntu Mate system onto the Philips USB
Then used a working Raspian SD card and altered the root=/dev/mmcblk0p2 line in the cmdline.txt file in the boot partition to reflect the partition in my Philips USB drive.
I left the Raspian partition intact (Did not delete) so that I was able to confirm booting the Ubuntu Mate from a working Raspian card merely by altering the root=/dev/sda1 instruction.
Time came to try; booted very fast, up came my Ubuntu Mate desktop quickly but from then on everything painfully slow, unusable in fact. Mouse still working fine with no jerkyness but desktop functions too slow to be of any use.
So, it looks like I am almost there but something is amiss.
Any clues or comments appreciated.