@lakotaubp okay I tried now anything. The Msm Downloader (show nothing). Pressing Vol- and Vol+ for more than 5 Sec. Pressing Vol+ and Power for some Secs. Pressing Vol+, Vol- and Power together for more than a min. Nothing happens at all. So now I connected the Phone again and in the device manager is now: "Remote NDIS based Internet Sharing Device". What?! How can this be?

Hey guys, I think now my OnePlus 6t is hard bricked. I installed Ubuntu Touch successfully, but now after some days I tried to boot it again, because I want to test it now. But for some reasons it booted and after this it was shortly blue (light blue) and than it gone dark. I tried to boot it again, powered it off, loaded it (still while loading it kept dark). In the MsmDownloader Tool I can't get it in. Whats happened? I thought it was still okay at all. I think I hard bricked it now, but how?! The installation was successfully done and I already installed succesfully apps.

@lakotaubp is now all fine with the OnePlus 6. I recovered it with the Qualcomm Emergency Downloader.

@mrt10001 how can I flash it back to stock rom? I am not able to find a device:

adb reboot bootloader
error: no devices/emulators found

Is my phone now bricked? What happened here? How can I install TWRP now? I am really not sure for now.

@mrt10001 for both commands the following output:

fastboot oem unlock_critical
FAILED (remote: '	Device already : unlocked!')
fastboot: error: Command failed

@lakotaubp Well i ran the command:

fastboot flashing unlock

FAILED (remote: '	Device already : unlocked!')
fastboot: error: Command failed

So why the fastboot is still telling me this?

fastboot oem device-info
(bootloader) Verity mode: true
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: false
(bootloader) Charger screen enabled: true
(bootloader) enable_dm_verity: true
(bootloader) have_console: false
(bootloader) selinux_type: SELINUX_TYPE_INVALID
(bootloader) boot_mode: NORMAL_MODE
(bootloader) kmemleak_detect: false
(bootloader) force_training: 0
(bootloader) mount_tempfs: 0
(bootloader) op_abl_version: 0x0
(bootloader) cal_rebootcount: 0x31
OKAY [  0.001s]
Finished. Total time: 0.001s

@keneda nah critical unlock is for updating the bootloader I guess. So my problem is now. The device won't be detected if I connect it within the recovery mode. With the fastboot is everything fine. So my problem for now is, I don't know why no Notebook (Windows, Mac oder Linux) can detect the phone while it is in recovery mode. And no it also won't be shown in the device manager in windows.

@keneda Okay in Fastboot mode there can the device be detected. If i switch into the recovery mode it is unable to detect the device. How can this happen?

With fast boot there is the following output:

(bootloader) Verity mode: true
(bootloader) Device unlocked: true
(bootloader) Device critical unlocked: false
(bootloader) Charger screen enabled: true
(bootloader) enable_dm_verity: true
(bootloader) have_console: false
(bootloader) selinux_type: SELINUX_TYPE_INVALID
(bootloader) boot_mode: NORMAL_MODE
(bootloader) kmemleak_detect: false
(bootloader) force_training: 0
(bootloader) mount_tempfs: 0
(bootloader) op_abl_version: 0x0
(bootloader) cal_rebootcount: 0x31
OKAY [  0.001s]
Finished. Total time: 0.002s

@lakotaubp yes it was also in Android 9, the recovery Mode thing can I test later, so that I can try to check if it is there. Do you know a good command on ubuntu to check if the device is there?

@keneda Yes, I just plugged the one OnePlus 6t off (from the same cable) and plugged the other OnePlus 6 in.