We're only nine days into 2019, and Microsoft has created the second 19H1 build of the year to Insiders on the Fast ring.

Build 18312 gives users access to the new 'reserved storage' feature which we covered yesterday. This reserves around 7GB of storage on Windows 10 systems to ensure day-to-day smooth running, and also to make sure that large updates don't fail on systems with limited free space. To try it though, you will have to tell you this quest that will provide you with access into it within the next flight. Should you run the quest before upgrading to this new build, you're going to get access to the feature when you do upgrade.

This new build also greatly enhances the per-process FLS (Fiber Local Storage) slot allocation ceiling for musicians, and introduces a brand new UI for Reset this PC as part of Settings > Update & Security > Recovery.

There's also new command line choices for the Windows Subsystem for Linux (WSL) command line tool. The changes include:

Consolidated command line options -- The wsl command line tool now includes choices to manage your WSL distros which are contained in the wslconfig command line tool. We plan to only update the wsl tool using the latest management options moving forward.
Import a distro for easy sideloading including to non-system drives -- Use the "-import" choice to imports a tar file like a new distribution. You are able to specify the distribution registry to the location of your choice including non-system drives.
Export your WSL distribution for simpler environment management -- Use the "-export" option to export a distribution to some tar file. Your distro will export to your default downloads location.

General changes, improvements, and fixes in this build include:

Fixed an issue in which the Network shares page in File Explorer used black text in dark theme.
Fixed a problem impacting Microsoft Edge reliability in the last few builds.
Fixed an issue where USB printers may appear twice in Devices and Printers under Control Panel.
Fixed an issue in which the console would lose color when spawning a new Command Prompt.
Following up on Microsoft's earlier try to result in the console scrollbar dark when dark theme is enabled, the title bar will also now bar dark (if you haven't chosen in Settings to use your accent color within the title bar).
Microsoft is adding an organization policy for disabling acrylic on the sign-in screen. This can be found in Group Policy Editor under Administrative Templates > System > Logon > "Show clear logon background".
Fixed an issue where custom desktop/Lock screen images deployed with a runtime provisioning package could be removed after Windows Update.
Fixed a problem where the shadow behind a flyout might zoom in following the flyout appeared.
Fixed an issue where, if enabled, the Windows Start Up Sound might not play after turning on from shutdown or after updating.
Fixed a problem in which the File Properties dialog wouldn't show the entire timestamp if "Conversational format" was selected for the Date Modified column in File Explorer.
Fixed a problem in which the hyperlink colors must be refined in Dark Mode in Sticky Notes if Insights were enabled.
If you were experiencing Sticky Notes stuck on Loading with Build 18309, check the Microsoft Store for Sticky Notes version 3.5.4.

Known issues to understand are:

The Windows Security app may show a mystery status for that Virus & threat protection area, or not refresh properly. This might occur after upgrade, restart, or settings changes.
Launching games that use anti-cheat software may trigger a bugcheck (GSOD).
Clicking your account in Cortana Permissions doesn't mentioning the UI to sign out from Cortana (if you were already signed in) for many users in this build.
Creative X-Fi sound cards are not working properly.
When attempting to update this build some S Mode devices will download and restart, but fail the update.
Night light functionality is impacted by a bug in this build. Microsoft is working on a fix, and it will be contained in a future build.
When you open Action Center the quick actions section may be missing.
Clicking the network button on the sign-in screen doesn't work.
Some text in the Windows Security app might not currently be correct, or might be missing. This might impact the ability to apply certain features, for example filtering Protection history.
Windows feature update may fail but appear like a successful update in Windows Update history page. Should this happen you will notice several successful install for the same update within the history page.
Users may see an alert their USB happens to be being used when attempting to eject it using File Explorer. To avoid this warning, close down all open File Explorer windows and eject USB media while using system tray by clicking on 'Safely Remove Hardware and Eject Media' and then selecting the drive to eject.
When performing Reset this PC and selecting Keep my files on the device that has Reserved Storage enabled the consumer will need to initiate an additional reboot to ensure Reserved Storage is working again properly.
There is a problem impacting a small amount of users enrolled in Microsoft Intune where they may not receive policies. Those coverage is not applied and left in "pending" state on the server. The workaround is to visit Settings/Accounts/Access school or work and "Disconnect" your Azure AD account after which re-enroll.