Yesterday, Microsoft pulled the latest Windows Technical Preview for phones (build 10051) for the Lumia 520, and its siblings including the Lumia 521, 525, and 526. The reason being that once users upgraded to Windows 10 and then tried to revert back to Windows Phone 8.1, those device would get bricked.
Microsoft has today issued a response on the Microsoft Answers forum detailing the underlying issues. While the company is still investigating, it has determined ones of the issues that is causing devices to get bricked. The software giant claims that the aforementioned devices were rejecting the recovery images being flashed due to data writing size and speed.
“The blocks of data were too large for some devices to handle, and the memory on the device was having trouble with the speed at which the data was being written. In short, devices were getting too much data, too quickly. This would cause the failure as the new software is corrupted.” – Microsoft
Microsoft claims that after monitoring the logs, it appears that only a small portion of Lumia 520-series device are affected. Unfortunately, there is no way for users to determine beforehand whether or not their device is susceptible to getting bricked.
As a result of this issue, Microsoft is changing the way the Windows Phone Recovery Tool performs the recovery procedure. This will hopefully result in less devices getting bricked in the future.
“We will be lowering the size of the data blocks being fed to the device during recovery (lowering from 2MB down to 128kB), and we will also be slowing down the data writing speed (from 8MB/sec to 5 MB/sec). Testing has showed promising results from these changes.”
A new version of the Recovery Tool has been released (version 1.2.4) and it includes the changes Microsoft detailed. Users with bricked devices are encouraged to attempt to recover again using the new version of the tool. Be sure to let Microsoft know whether it worked for you.