While Windows 10 Mobile build 10581 makes great strides in fixing bugs and issues in anticipation for the upcoming RTM release, there are still known issues remaining. Among them include the inability for insiders to set their default save locations through Settings > System > Storage. Users using SD cards will see messed up names in Storage.
A community discussion post further elaborates on this issue:
- A hard reset on the new build will revert back to the factory default of "on phone", and cannot be changed this build.
- Users who upgrade from an earlier build to 10581 will have their current configurations respected.
- Users who have previously set their default save locations to SD cards will still be able to access their data.
As such, the only issue seems to be build 10581's inability to change the default save locations but does not affect its actual functioning.
The Windows team is addressing as many configuration concerns as it can in its community discussion forum. If you have any questions or concerns about storage maintenance on the last build, head over there for some possible answers.