Rebuilding TS-231P. Should I complete it?

[GV231]

I have a QNAP TS-231P with two HDDs installed:
1. Seagate IronWolf ST8000VN0022 (8Tb). Drive number one according to schema on top, the one closer to PCB and On/Off button
2. Toshiba N300 HDWN180 (8Tb). Drive number two according to schema on top, the one further from PCB and On/Off button

The drives are in mirror and contain encrypted (locked) by QTS software. The key is not saved and entered manually.

6.6.2021 One of them has failed (was in disconnected state). I noticed this when switching NAS off. Since then NAS has been in off state.
22.6.2021 I switched on the NAS and noticed that both HDDs are in connected state and Rebuild process has started automatically (without entering encryption key).

The process wanted to make rebuild for 14h. At 3.5% I decided it makes no sense and switched device off (using QTS UI in browser and choosing to shutdown).

I already bought replacement HDD (will arrive tomorrow), but I need assistance to what to do next.

I am first time in such situation and have a bunch of questions:
1. How to make sure which HDD exactly failed?
2. Is it safe to switch device on to make sure which HDD exactly has failed? Whether it will continue to rebuilding the raid?
3. Can I switch NAS with one of the mirror HDDs in bay only?
4. What would happen if I switch on NAS with only a failed HDD in bay? Whether it will be marked as a good one and will become "primary" (i.e. disk with newest data?)

Any assistance is welcome.

Thank you.

[GV231]

Anyone? Any hints?

[dolbyman]

with two different HDD models, the ID should be easy

find out what HDD has IO/SMART issues and check the disk overview what slot contains the disk model in question (it will be visually shown in the webgui)