Windows 10 Universal App File/Directory Access

In UWP apps, you can only access the following files and folders:

• Directories which are declared in the manifest file (e.g. Documents, Pictures, Videos folder)
• Directories and files which the user manually selected with the FileOpenPicker or FolderPicker
• Files from the FutureAccessList or MostRecentlyUsedList
• Files which are opened with a file extension association or via sharing

If you need access to all files in D:\, the user must manually pick the D:\ drive using the FolderPicker, then you have access to everything in this drive…

UPDATE:

Windows 10 build 17134 (2018 April Update, version 1803) added additional file system access capabilities for UWP apps:

• Any UWP app (either a regular windowed app or a console app) that declares an AppExecutionAlias is now granted implicit access to the files and folders in the current working directory and downward, when it’s activated from a command line. The current working directory is from whatever file-system location the user chooses to execute your AppExecutionAlias.

• The new broadFileSystemAccess capability grants apps the same access to the file system as the user who is currently running the app without file-picker style prompts. This access can be set in the manifest in the following manner:

    xmlns:rescap="http://schemas.microsoft.com/appx/manifest/foundation/windows10/restrictedcapabilities"
    ...
    IgnorableNamespaces="uap mp uap5 rescap">
    ...
    <Capabilities>
      <rescap:Capability Name="broadFileSystemAccess" />
    </Capabilities>

These changes and their intention are discussed at length in the MSDN Magazine article titled Universal Windows Platform – Closing UWP-Win32 Gaps. The articles notes the following:

If you declare any restricted capability, this triggers additional
scrutiny at the time you submit your package to the Store for
publication. … You don’t need an AppExecutionAlias if you have this
capability. Because this is such a powerful feature, Microsoft will
grant the capability only if the app developer provides compelling
reasons for the request, a description of how this will be used, and
an explanation of how this benefits the user.

further:

If you declare the broadFileSystemAccess capability, you don’t need to
declare any of the more narrowly scoped file-system capabilities
(Documents, Pictures or Videos); indeed, an app must not declare both
broadFileSystemAccess and any of the other three file-system
capabilities.

finally:

Even after the app has been granted the capability, there’s also a
runtime check, because this constitutes a privacy concern for the
user. Just like other privacy issues, the app will trigger a
user-consent prompt on first use. If the user chooses to deny
permission, the app must be resilient to this.