Custom ROM on B slot

Hi all,

I just received my FP3+ I would like to install a custom ROM, iodéOS, on my device. I already did a lot of research but I’m not sure about every step. I just don’t want to brick my brand new device. During my research I learned that FP3+ has an A and B flash slot. Now would it be possible to flash the unused slot with iodéOS and leave the other with FairphoneOS on it as it is?

I found a something to flash both slots with the stock ROM so I assume it will be possible. However the install guide iodéOS provided is quite vague for someone who has never flashed a ROM.

Currently I think I will need to do the following:

  • Using the guide to install LineageOS I should follow all steps up until step 5.

  • Now, I know I will have to execute “fastboot set_active other” and I believe it’s best to execute that now, before I boot into recovery?

  • Then I just execute step 5, following the LineageOS recovery instructions but use the iodéOS recovery.
    Will this be okay?

Now, to be able to switch without my PC I can use TWRP. From what I read, I should first boot into TWRP then flash it, else I will break things, correct?

If I manage to basically dual boot my phone, system updates will brake the other install so should I never update, just manually re-flash the new version so I don’t break things?
I also understood that there’s only one DATA partition, will this be a problem as I’m using two different ROMs? I don’t plan on using the FairphoneOS a lot beside a 1 or 2 apps if they don’t work on iodéOS, I just like to still have it in case something goes wrong. They are banking apps that do not function on rooted phones and I’m afraid if I flash over the FairphoneOS ROM it might detect it and I don’t want to actually root it and use Magisk to hide it (although I heard from a friend it will work that way. Thank makes me think, can I root only the iodéOS ROM if necessary?).

Sorry for the long post, I just want to be sure about the steps I take because I don’t want to brick my device. The battery of my Microsoft Lumia 950 XL is too bad so I needed a new phone. I don’t want to join the Apple ecosystem and I don’t like Google knowing everything so I hope my FP3+ with a custom ROM will get me going for another 5 or more years. (In case anybody is interested on using a Lumia in 2022, let me know)

Thank you in advance.


Welcome to the community forum.

Theoretically, yes.

Theoretically, yes.

Yes, that’s where the theoretical dual boot breaks in practice.

It has been observed that an OS might not like the state the data partition is in because of booting an OS in the other slot, with the only way out being a factory reset = wiping the data partition.
Game over.


ohn, that’s unfortunate. If I flash only one, can I then re-flash FairphoneOS without any root issues?

What do you mean with root issues?

Apps detecting root. I don’t know why I was so worried about that since I haven’t rooted.

Anyway, I flashed iodéOS and everything seems to work. It was probably the scariest thing I have done in a long time.

Thank you for the help!


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