Out of interest, I'm trying out Ubuntu on my Raspberry Pi 3B+.
I've downloaded the image from
I've copied this file to the Pi running Raspian and confirmed that sha256sum produces a checksum which matches the published one - I wasn't confident since the download stalled overnight and I had to resume it where it left off. And on a 2 Mbps connection (yuk!) I wasn't keen to start it all over again. 1.2 GB at 2 Mbps takes about 2 hours.
I've expanded the .img.xz file to .img using 7Zip on Windows 7 and then used both Rufus and W32 Disk Imager on Windows and ddrescue on Raspian to write this image to a 32 GB SanDisk Ultra micro SD card. Using W32 Disk Imager I've also verified that the SD card and the .img file match.
But when I try to boot from that SD card (replacing the Raspian card in the Raspberry Pi) it stays on the multi-coloured spectrum screen, with a lightning symbol in the top right hand corner. I've checked and the 5V power rail on the GPIO connector is 5.05 V - so it's a false alarm. The power light flashes: 4 long flashes, 4 short flashes - repeatedly. It seems that a blinking power light (red) indicates a PSU problem. but it's 2.5A, certified for RPi, and as I say, the measured voltage is just over 5V. And it boots fine to Raspian on another identical make/model of card.
Any suggestions? Is it some sort of incompatibility between an image that is said to be good for RP2 and RP3, and my RP3 B+?