If ever there was proof that serial (and thus USB) comms and firmware flashing was risky, it comes as Microsoft has had to update its Windows Phone Recovery Tool (WPRT) to send over data in smaller chunks, after a rash of people failing to restore their low end Lumia 520s and 521s after trying the Windows 10 Insiders build. It seems as though the slower chipsets in some affected devices couldn't keep up with the flashing data stream and ended up inoperative. The fixed tool should mean that flashing is now successful - users may have to use a 'dead USB' flash if they see "My phone was not detected", but it should all now work. There have been some reports of other Lumias affected by the issue, but Microsoft is still investigating - there are many reasons why a PC wouldn't be able to communicate perfectly over USB with an accessory - this is Windows, after all, and it's a complex system.

From the Microsoft discussion forum:

For users with Lumia 520/521 devices that have become unusable after an attempted recovery using the WPRT:
 
Failures:
Devices that have failed to recover appear to fall into one of two different states:

• Displaying a red Nokia logo
• Displaying a black screen and the device is non-responsive after attempting to boot.

Issue Identification:
Although our investigation is not complete, we’d like to share some initial insight into what we’ve uncovered:
 
Data writing size & speed:  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.
 
Changes:
As a result of these findings, we are making two key changes to the way the recovery tool (WPRT) works.  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.
 
User Expectations & Recovery efforts:
For users who have already unusable devices, we’re continuing to research this situation.  There is no guarantee that using this updated version of the tool will recover an already-bricked device.  If your device is showing the red Nokia logo, please do the following:

• 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

If your device is on a blank/black screen and is unresponsive, you may attempt the same steps as listed above.  Devices that do not have the red Nokia logo on the screen are in a different state and will not have the same recovery rate should they be recoverable.  Please do try to recover and share your results.

Only a small proportion of 520/521 devices are affected, interestingly, so it's probably a set that had a specific flash memory chip from a particular factory and/or batch. Microsoft does note that 'there is no direct way for an end-user to check their device unfortunately'.

As a consequence of all this, the Windows 10 Insiders provisioning has been temporarily halted for the Lumia 520 and 521.

Have you been hit by any of this and which device was affected?