Microsoft today released a brand new Windows 10 preview with improvements to disk space management, Reset this PC, and the Windows Subsystem for Linux (WSL). The update bumps Windows 10 from build 18309 (made available to testers on January 3) to build 18312. These builds come from the 19H1 branch, which represents the Windows 10 update which will get to the first 1 / 2 of this year.

Windows 10 is being developed like a service, meaning it receives additional features regularly. Microsoft has released six major updates to date: November Update, Anniversary Update, Creators Update, Fall Creators Update, April 2018 Update, and October 2018 Update.

The seventh update can change how Windows 10 manages disk space. Some disk space (about 7GB) will be set aside as "reserved storage" for updates, apps, temporary files, and system caches. The aim, Microsoft says, is to ensure critical OS functions always have access to disk space. Reserved storage are only introduced automatically on PCs that come with 19H1 pre-installed or on PCs where 19H1 was clean installed. You can try reserved storage about this build by running this quest.

Next, Microsoft has added new UI for "Reset this PC" (Settings => Update & Security => Recovery). The new UI requires fewer clicks and it is more consistent across devices with various configurations.

This build also adds new command line choices to the WSL command line tool (wsl.exe). Among the changes (release notes) you may expect consolidated command line options, the option to import a distro for simple sideloading, including to non-system drives, and also the option to export your WSL distribution for simpler environment management.

Lastly, this build raises that per-process Fiber Local Storage (FLS) slot allocation ceiling. This really is helpful for musicians who hit the FLS limit, which prevented them from loading many plugins into their DAWs (Digital Audio Workstations). The modification will also positively impact any application that dynamically loads hundreds or thousands of unique DLLs that have statically linked Visual C++ runtimes.
Bug fixes and known issues

This 19H1 build includes the following general bug fixes and enhancements:

Fixed a problem 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 a problem 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 on our work the 2009 release to make 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 in the title bar).
Added a group policy for disabling acrylic around the sign-in screen. This is often found in Group Policy Editor under Administrative Templates > System > Logon > "Show clear logon background".
Fixed an issue where custom desktop/Lock screen images deployed by a runtime provisioning package would be removed after Windows Update.
Fixed a problem where the shadow behind a flyout might focus following the flyout appeared.
Fixed a problem where, if enabled, the Windows Start Up Sound may not play after turning on from shutdown or after updating.
Fixed an issue 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 an issue in which the hyperlink colors must be refined in Dark Mode in Sticky Notes if Insights were enabled.
If you had been experiencing Sticky Notes stuck on Loading with Build 18309, check the Microsoft Store for Sticky Notes version 3.5.4.

This build has 16 known issues:

The Windows Security app may show an unknown status for that Virus & threat protection area, or otherwise refresh properly. This might occur after upgrade, restart, or settings changes.
Launching games which use anti-cheat software may trigger a bugcheck (GSOD).
Clicking your account in Cortana Permissions doesn't bringing up the UI to sign from Cortana (should you be already signed in) for many users in this build.
Creative X-Fi sound cards aren't working properly.
When trying to update this build some S Mode devices will download and restart, but fail the update.
Night light functionality is influenced by a bug in this 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 within the Windows Security app may not currently be correct, or might be missing. This might impact the capability to use some features, such as filtering Protection history.
Windows feature update may fail but appear as a successful update in Windows Update history page. If this happens you will notice several successful install for the same update within the history page.
Users may see a warning their USB happens to be being used when attempting to eject it using File Explorer. To avert this warning, close down all open File Explorer windows and eject USB media while using system tray by hitting 'Safely Remove Hardware and Eject Media' after which 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 an issue impacting a small amount of users signed up for Microsoft Intune where they might not receive policies. Those policies are not applied and left in "pending" state on the server. The workaround is to visit Settings/Accounts/Access work or school and "Disconnect" your Azure AD account after which re-enroll.
Windows Sandbox doesn't load on PCs with multiple GPUs.
In certain cases, launching Centennial apps will cause a bindflt.sys bugcheck (GSOD).
If you put in the recent builds from the Fast ring and switch to the Slow ring - optional content such as enabling developer mode will fail. You will have to remain in the short ring to add/install/enable optional content. It is because optional content is only going to install on builds approved for specific rings.

Of course, don't install this on your production machine.