I have installed Windows using Boot Camp, but cannot access the partition from macOS!
The most common issue when trying to access the NTFS volume that contains your Windows installation is the use of Windows' Fast Startup and hibernation features. Using them results in the creation of a hibernation file on the volume that prevents mounting it externally.
NTFS for Mac will typically ask a user to purge the hibernation state when attempting to mount the filesystem unless Disable recovery dialog for hibernated volumes-setting is toggled in System Preferences. Doing so may result in loss of data if you haven't saved your files prior to hibernating.
We recommend shutting down Windows properly before attempting to mount the filesystem from within macOS. We also recommend disabling Fast Startup.
After successfully mounting the Windows volume, I can no longer choose to start Windows from Startup Disk!
This is normal. You need to unmount the volume, before it can be set as your startup disk.