Microsoft will make a couple of changes to its Windows Phone Recovery Tool a few days after it was discovered to be the cause of bricking devices like the Lumia 520 when rolling back from the Windows 10 for phones preview.
Specifically for the Lumia 520 and its 521 variant, Microsoft said:
" It was discovered that some devices are having trouble accepting the recovery image data being flashed. 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."
The issue is only affecting a small number of those smartphones but Microsoft said there's no way for owners to check if their phone is part of that subset with the issue. Microsoft then outlined their plans for the changes to the recovery tool:
"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."
"To implement these changes, we have released an updated version of the WPRT tool. You may notice when launching the WPRT that it checks for updates automatically. Before attempting any further recovery or flashing on 520/521 devices, ensure that you've closed and re-launched the WPRT. It will find this new mandatory version and automatically update."
Microsoft has pulled the update path for Windows 10 for phones preview for the affected Lumia products, and the company now plans to contact some phone owners to see if this solution will solve this bricking problem. It added, "Once we're confident in the results, we'll turn the update path back on for these devices". Microsoft also said it will continue to do research for phone owners who have had their devices bricked but added, "There is no guarantee that using this updated version of the tool will recover an already-bricked device."