You don’t have to do anything because this issue will be automatically resolved the next time you plug in the hard drive.
If you add new music or movies during the unplugged period, you can always consolidate those files to the external storage later.
Then an exclamation mark will appear next to the item.
If you still keep the items that you want to play inside the local drive, click ““.
You are also able to create sub-folders and place imported media from Music to Audiobooks into them. Ensure that the metadata is working well for you and has, at the very least, correct info.