Microsoft has released a possible fix for low-end Lumia smartphones that have been bricked after installing an early Windows 10 preview build.

It seems a shortage of memory in the Lumia 520 and Lumia 525 is behind the bricking of some of the units, after owners installed a Windows 10 Technical preview build, named 10051, released through Microsoft's tester Fast lane.

In an update on its Windows Insider Program site on Friday, Microsoft said only a "small portion" of Lumia 520 and 521 devices were affected.

Microsoft last week pulled down the test build for the Lumia 520, 525, and 526, following reports that some of the handsets were becoming unusable, typically after using Microsoft's Windows Phone Recovery Tool (WPRT) to roll back to Windows Phone 8.1. Affected devices were left with "no direct way for an end-user to check their device", Microsoft said.

According to the company's initial investigation, failed devices displayed either a red Nokia logo or a black screen and would be otherwise non-responsive.

The build for these devices remains unavailable, but Microsoft seems to have found the source of the problem: its recovery tool was moving data too quickly for the low-end Lumia devices.

"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," Microsoft said.

The good news, according to Microsoft, is that not all units of those models are likely to flake out.

Microsoft has now released slower version of WPRT (version 1.2.4) that the devices can handle.

"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," Microsoft said.

However, even with the updated recovery tools, there is a caveat. "There is no guarantee that using this updated version of the tool will recover an already-bricked device," Microsoft said.

Also the recovery process may be different for devices displaying the red Nokia logo versus ones with a black screen. For devices in the former category, Microsoft advises users to:

Ensure your device has a good power source (connected via USB cable that can transfer data and charge the device simultaneously).

Close the WPRT, re-open, and take the update to the new version of the WPRT. The new version is 1.2.4.

Once he new version is installed, re-attempt the flashing process.

If your device is not recognized, click the "My phone was not detected" button and continue the attempted recovery process.

The company won't be re-releasing the technical preview build for the affected devices until its completed additional real-world testing.

"Once we're confident in the results, we'll turn the update path back on for these devices," it said.

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