Fix Offered for Some Lumias That Were Bricked by Windows 10 Technical Preview 2

I haven’t experienced this myself, but it’s not hard to imagine: You decided to install the latest Windows 10 Technical Preview 2 build for phones on a low-end Lumia handset only to discover that the build is unusable. So you revert to Windows Phone 8.1 using Microsoft’s recovery tool … and completely brick your phone. But there’s some good news: Microsoft has fixed the problem, assuming you’re using a Lumia 520 or Lumia 521.

Reports about bricked Lumia 520/521 handsets started circulating in the wake of the release of Windows 10 Technical Preview 2 for phones build 10051 about ten days ago (or nine days ago, if you had the kind of day one experience I did). But it wasn’t the new build that bricked the phones, it was a Microsoft recovery tool called the Windows Phone Recovery Tool. You use this tool to pretend Windows 10 never happened and go back to the most recent supported version of Windows Phone (typically some version of Windows Phone 8.1 these days).

I documented the Windows Phone Recovery Tool—and a related Lumia Recovery Tool—Windows 10 Technical Preview 2 for Phones Tip: Return to Windows Phone 8.1, not coincidentally back in February when Microsoft shipped the first pre-release Windows 10 build for phones. (Build 10051 is only the second such build.) Then, as now, many people who take the plunge instantly regret it and just want to go back to a clean, fully-working and supported Windows Phone OS. Understandable.

And generally speaking, the Windows Phone Recovery Tool works as advertised. But apparently it has some issues with low-memory devices like the 520 and 521. And in the words of Microsoft support, it was simply trying to feed those phones with data too quickly.

“It was discovered that some devices are having trouble accepting the recovery image data being flashed [by the recovery tool],” Microsoft notes. “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 good news is that this issue didn’t impact all Lumia 520/521 handsets, so if yours reverted just fine and you’re wondering what all the fuss is about, you were simply lucky. But if you did brick your phone—those that “failed to recover” (i.e. “were bricked”) display a red Nokia logo or a black screen on an otherwise non-responsive device—well, good news. It’s been fixed.

“As a result of these findings, we are making two key changes to the way the recovery tool works,” Microsoft explains. “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 get this change, you need the latest version of the Windows Phone Recovery Tool. If you already have this tool, it will snag the update the next time you run it. So try again with your bricked phone and you should be good to go. Assuming it’s a Lumia 520 or 521, that is.

Microsoft has also removed the Lumia 520 and 521 from the list of devices that can receive Windows 10 Technical Preview for phones until it can be sure this recovery tool update works for everyone.