Pre-orders for the HTC U Ultra don’t even start shipping in the U.S. until March, but TWRP is already available for the device. However, it’s not an official TWRP version but a beta ported over by recognized XDA Developer @Captain_Throwback.
As such, there are no guarantees that the custom recovery will work exactly as intended, but it will allow you to get TWRP 3.0.3-1 on your U Ultra the minute you unbox it. As it stands, the only known issue is that the reboot recovery option is not yet functional, but this will be fixed once the dev has access to the kernel source.
As always, there’s a lot of reading material over on XDA if you’re interested, so be sure to read up before proceeding. As always, if you’re using your U Ultra for a while before taking the plunge, be sure to back up any essential data first.
Flash in stock 6.0 firmware;
boot into fastboot mode and load TWRP (3.0.2) with /system READ ONLY mode. I think this is the cause for the failure;
Install SuperSU (2.7.9-SR3). And saw a message "System-less mode. Boot image support required." on terminal. What does "boot image support reqiured" means?
Reboot and SuperSU works like a charm.
Update: I've tried to boot TWRP without /system write protection and flash in SuperSU 2.79-SR3 successfully. So only the SuperSU is responsible for this issue?
The Pixel smartphones' new partition system and boot images have been a hot mess for developers and tinkerers who like to push their devices beyond the specs written on the shipping box. But even though this has slowed down the release of custom recoveries and other mods, it hasn't completely stopped our beloved enterprising developers who probably thought of the whole situation as a nice challenge instead of an unsurmountable obstacle.
Just yesterday, Ethan Yonker (Dees Troy) released an early alpha of custom recovery TWRP for the Pixel devices, but that created a problem for those who were using the boot-to-root images made by Chainfire for the Pixels. The devices would suddenly get encrypted after flashing SuperSU, which isn't something you want to happen spontaneously on your phone. Edit: actually, as Ethan corrected us, "the incompatibility comes from Chainfire using the recovery ramdisk to achieve system-less root on the Pixel devices. He would modify the kernel to always boot the ramdisk and used a modified init binary. TWRP also needed a modified init binary to achieve decrypt of FBE. The conflict occurs with the init binary." Chainfire explains that he and Ethan already anticipated the problem so he is able to release a fix quickly.
Long story short (and plenty of technical jargon that I would like to understand but don't have enough time to do so right now), SuperSU and TWRP now play nice together. There are lots of things to take into consideration and steps to follow closely to make sure you end up with a working device, so head over to Chainfire's post at the source link below. He also has all the download links you need.
Which version of Magisk do you use? Magisk 7.0 (I think) was not compatible with SuperSU, as it has phh superuser included and disabled SuperSU. I would recommand looking for the Magisk version that is compatible with SuperSU. In addition, SUhide can also make Android Pay work, you can try that out