Owners of the T-Mobile-exclusive OnePlus 7T Pro 5G McLaren interested in bootloader unlocking the device for root or ROM purposes should think twice before they do. Based on recent reports, that process can prevent the phone from being able to download future official updates, including one that was just released.

As well documented in this blog post, the issue stems from the fact that unlocking the device's bootloader (which requires a carrier unlock) triggers a full wipe of the userdata partition, deleting an essential file stored there and which is needed for updates to be installed. This may be part of the "master reset with hardware keys" process documented here (more commonly known as a "factory reset" via recovery). I should note that manually performing the maneuver described there in recovery outside of the bootloader unlocking process doesn't appear to break things in testing, though wiping via fastboot does.

Both unlocking the boorloader and wiping the phone via fastboot wipes the userdata partition, removing a symlinked image file containing a few OnePlus apps stored in that partition. This file by itself isn't necessary for the phone to work, but it is unfortunately required for updates to be correctly installed. In essence, the phone freaks out when applying so-called "delta" incremental updates if it thinks it has been tampered with. Removing that file convinces the phone that that has happened, breaking the ability to install these delta updates.

Since unlocking the bootloader triggers a wipe of the userdata partition as a security measure, that means unlocking the bootloader will prevent the phone from correctly downloading and installing updates.

An update failing due to the process described. Image: OnePlus Forums

By itself, this wouldn't be an issue for the root and ROM crowd, who are often used to inconvenient workarounds for software limitations, except for the fact that the missing file isn't publicly available and full images to restore a fully working system — usually the last-ditch effort for fixing a serious problem like this — aren't available. If you unlock the bootloader of your OnePlus 7T Pro 5G McLaren right now, or perform a "master reset" via recovery, you lose the ability to install updates, at least for the meantime.

We've reached out to both OnePlus and T-Mobile for more information, as well as details regarding how each plans to fix the problem and when, but a response wasn't immediately forthcoming. Some of those affected by this problem have been able to successfully securer device replacements from T-Mobile.

In the meantime, I urge device owners not to attempt a bootloader unlock or wipe their devices via recovery. We hope, at least, that the in-software "master reset"/factory reset doesn't cause the same issue, though I assume reports would be more widespread if it did.

Developers may have made a fix

Although neither T-Mobile nor OnePlus has addressed this problem, which prevents bootloader-unlocked OnePlus 7T Pro 5G McLarens from installing updates, developers have come up with a homebrew fix: They have successfully extracted the required image from the userdata partition, allowing those with an unlocked bootloader to restore it once it has been removed by the unlocking process, and downloads for the image (as well as an incremental update for those stuck on an earlier OS version) are now available.

While this should fix the problem, it can't really be tested until a new OTA update starts to roll out, which might take a while. But based on the facts known so far, it should present a possible (if obnoxious) solution, even if a direct fix from OnePlus/T-Mobile would be better.

For a bit of entertainment, you can read up on the full, dramatized process developers went though trying to find this fix here, and our original coverage of this story is just below.

A new OTA has landed, and the above fix has been confirmed as working for it. For those that run into this issue in the future, this fix should work, even though T-Mobile and OnePlus have yet to release factory images for the phone at the time of writing.