Well, it’s also worth mentioning that even after wiping the system partition and flashing the Stock ROM on the active partition, the inactive slot still might have the OnePlus 9 Pro firmware installed. Again booting into the OS for the first time may take a couple of minutes.
Once done, tap on Install > Browse and select the downloaded firmware file > Swipe the slider to confirm flashing the firmware.Then go to Wipe again from the TWRP menu > Tap on Advanced Wipe > Select Dalvik/ART Cache and System > Swipe the slider to wipe.Once the TWRP menu appears, go to Wipe > Select Format Data > Type YES > Swipe the slider to Factory Reset.Now, boot your OnePlus 9/9 Pro device into the TWRP Recovery Mode.Wait for it to complete and your device will reboot to the system automatically.
How to Upgrade to OnePlus 9 Firmware Switching from OnePlus 9 (Pro) Firmware Finally, your device will boot into the OS.
Well, you’ll need to use the MSM Download Tool (Indian Variant) that has been designed for the OnePlus 9 Pro model.
Install Qualcomm HS-USB QDLoader 9008 Drivers Fix: Unable to Recognize OnePlus 9 or 9 Pro Using MSM Tool: Sahara Connection Error.