You can have a disk that is mounted in your filesystem, e.g.:
/mnt/disk
And under this path are all the files/folder mounted from this drive. E.g. if that is the Nextcloud data folder, there is at least the .ocdata file.
If the disk is unmounted, there is still the
/mnt/disk
folder which is empty. Now Nextcloud could think, that suddenly all the files were deleted. For that reason, if there isn’t the .ocdata file, it knows that something is wrong and it doesn’t see the real data folder.
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