[Xiaomi Redmi Note 4 Questions & Answers] edwardenglish:
OK, I tried to install the latest stock ROM with stock recovery but it has version 3.0 installed that has no flash zip option anymore (you can google it).

So I tried to install TWRP TWRP 3.1.1-0 and TWRP 3.1.0-0 from official homepage both fail to install the update.zip file saying in log

unmount of /firmware failed; no such volume
unmount of /firmware failed; no such volume
mount: failed to mount /dev/block/bootdevice/by-name/sec at /firmware: Invalid argument
mount: failed to mount /dev/block/bootdevice/by-name/sec at /firmware: Invalid argumentMountFn took 00s.

MountFn took 00s.
package_extract_file: can't open /firmware/image/sec.dat for write: No such file or directory
package_extract_file took 00s.
package_extract_file took 00s.
unmount of /firmware failed; no such volume
unmount of /firmware failed; no such volume
package_extract_file took 00s.
package_extract_file took 00s.
SetMetadataFn took 00s.
SetMetadataFn took 00s.
about to run program [/cache/miui_update] with 1 args
[RELINK] Relink ...
RunProgramFn took 00s.
RunProgramFn took 00s.
Patching system image unconditionally...
performing update
Patching system image unconditionally...
package_extract_file took 00s.
package_extract_file took 00s.
blockimg version is 4
maximum stash entries 0
creating stash /cache/recovery/2bdde8504898ccfcd2c59f20bb8c9c25f73bb524/
254492672 bytes free on /cache (0 needed)
erasing 184138 blocks
writing 576394 blocks of new data
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 1024 blocks
zeroing 300 blocks
wrote 602294 blocks; expected 602294
stashed 0 blocks
max alloc needed was 4096
deleting stash 2bdde8504898ccfcd2c59f20bb8c9c25f73bb524
bytes_written_system: 2466996224
bytes_stashed_system: 0
Perform Block Image Update took 33s.
unmounting /cust succeeded
script succeeded: result was [1.000000]

Can anybody help me? What can I do?

EDIT (Problem solved)

OK weird thing. After I tried several times, always getting the same old "error" shown above I grew bold and tried to reboot no matter what. TWRP warned me: there is no system installed. Do you really want to reboot? I found myself in awe few seconds later because there was no bootloop. I just came back into the system running the latest update. So I dont know exactly what those errors mean and why TWRP wouldn't detect the system being installed.

Thanks for all of your help and maybe someone else has this issue too, so dont be afraid

Summary for installing latest update on rooted unlocked device:

1. download zip from xiaomi homepage

2. place it on device storage

3. head into custom recovery TWRP

3a. take a backup - helped me!!

4. wipe system & cache (I kept Data, no problem so far)

5. install the update.zip, dont mind if you get the error from above or if TWRP tells you that there is no OS installed

6. reboot into system let it settle

7. shutdown and reinstall TWRP via ADB

8. launch TWRP again and flash superSU

updated and rerooted =)