Just a few days back Microsoft, pulled Windows 10 for phones preview for Lumia 520, 525 and 526 due to instability caused when attempting recovery using the Windows Phone Recovery Tool. The reason behind the issues as said by Microsoft:
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.
Even tho the issues is being encountered in few devices, Microsoft is taking action and making some changes in the Windows Phone Recovery Tool (WPRT). Microsoft sheds light on the changes highlighting:
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.
So if you had problems with your Lumia too, now you can try to close the WPRT, re-open, and take the update to the new version of the WPRT i.e. 1.2.4. Once you have the new version installed in your system, you can continue with re-attempting the flashing process. And hopefully all will go well…