let me note some things on installing CrDroid / LineageOS with TWRP alpha (06/2021).
This might guide some newcomers.
TWRP alpha seems to be unable to decrypt or mount the internal storage as writeable if NO rom has been installed before or partitions have been wiped and formated. (with MTP enabled in TWRP your computer will find the phone and internal storage, but it will fail to copy files)
IF a rom has been installed before, it tries to decrypt FBE (file based encryption) with the PIN you chose and then it is possible to load files with MTP to internal storage.
So, if you start from scratch, you have no ROM installed before, or wiped and formatted everything:
you fastboot-load TWRP and end up not being able to upload files (like ROM.zip or TWRP.img) with MTP to the internal storage... how do you select the files for flashing if they aren't available?
1) use external sd card (copy files on ext. sd card, select files from there in twrp)
2) use usb-otg if you have one.
or, and that's my way to go if you don't have a phone with ext.sdcard or usb-otg:
3) use ADB sideload.
in TWRP use ADB sideload to Flash the ROM.zip first (CrDroid/LineageOS). Start the ROM, create a PIN. That way the encryption password is set.
Now start TWRP again with fastboot boot twrp.img (because currently the rom-recovery is default).
this time it asks for user password, enter your pin/pattern and TWRP is finally able to decrypt partitions and use the internal storage. from that point on forward you can copy images with MTP to your phones internal storage.
now in TWRP with MTP enabled, copy twrp.img to internal storage and install the twrp.img as ramdisk. don't hit the reboot (to system) button right there, go back and choose reboot-> reboot to recovery first.
After that it's all done, boot to system. The ROM and TWRP recovery is installed, and TWRP is finally able to decrypt storage and partitions.
on a sidenote:
- TWRP a/b switching indeed isn't working, use fastboot.exe --set-active=a (or b)
- if you flash magisk in twrp, *don't* install TWRP to ramdisk again right after that, just reboot to system, or it gets overwritten again.
Have something similar here, but in the system.
Did a fresh install (directly after unlocking) of CrDroid via TWRP / ADB sideload and installed Open Gapps.
When the system has booted, computer will find the phone and internal storage, but it will fail to copy files. Internal storage seems to be mounted read-only or something.