Windows 10 Redstone 4 (Spring Creators Update) includes ReFS 3.4
The upcoming spring release for Windows 10 (Redstone 4) will include ReFS 3.4 (ReFS = Resilient File System)
When you install one of the current Insider Builds (17074, 17083) all your partitions and volumes (and not only storage pools) will be updated, If you want to go back to build 1709 or older (like I did because of a driver issue with my test system) your partitions will no longer be recognized and they be will be displayed as RAW volumes. This has also happened to others with external drives.
So be careful with upgrading to these Insider versions or the upcoming Spring release that includes changing and updating file systems if you want to be able to switch between Windows 10 builds or expect you might need to go back to an older build for compatibility reasons.
If you run into this issue the best way to get your data back is to go back to the Insider build, I went back to 17074 to restore access to a partition holding my data.