On November 18, 2015 2:00 pm (Seattle Time) Windows 10 Mobile IP Build 10586 was released to Windows Insiders on the Fast Ring .

This build carried a lots of bug fixes and improvements over the last build (10581) .

According to Microsoft, some of the fixed bugs are :

• They have fixed the issue that caused the Start experience to become corrupted (garbled) after upgrading and restoring a backup from a phone that had a different resolution.
• You should now be able to set your default save locations via Settings > System > Storage and it will correctly reflect the current storage settings. Your SD card should no longer show up as a garbled name in Storage settings either.
• Moving apps to a SD card should no longer cause those apps to crash.
• They have made improvements to the Messaging + Skype app – through both app updates via the Store and also on the server side.
• This should be seeing less resuming/loading text when switching apps.
• The physical camera button should be working again if it wasn’t working for you before.
• Apps and games should download from the Store more reliably.This is a good sign of Windows 10 Mobile hitting RTM and the OS being free from Major bugs like that Storage not reflecting issue . 

• After Build 10581 was released to Windows Insiders in the Fast ring, they discovered a bug in the build that will cause the filesystem to become partially corrupted after doing a factory reset. For those who did a factory reset of their phone, you most likely did not notice this issue on Build 10581. Due to this bug however, upgrading to Build 10586 will cause your phone to go into a reboot loop after the upgrade completes – rebooting at the Windows or operator logo. To recover your phone, you can use the hardware key combination to reset your phone which will then put it at the OOBE experience on Build 10586. They highly recommended that you make sure you do a backup your phone *before* upgrading to Build 10586 due to this bug. Additionally, you can also recover your phone by using the Windows Device Recovery Tool to go back to Windows Phone 8.1 and then upgrade to Build 10586.
• Deploying Silverlight apps through Visual Studio to your phone still won’t work in this build. This issue will be fixed with the release of Visual Studio 2015 Update 1 on Nov. 30^th. You can deploy UWP apps to your phone without any issues.
• There is a known issue where the tile for Insider Hub still remains under All apps but doesn’t open. Insider Hub isn’t included in this build. There isn’t a way to get it back unfortunately. However, it’ll return in a future flight! In the meantime, use Insider Hub on the PC as a workaround.