My Plex server decided to go through my database and screw with all the posters and art, so I made the below script which can be used to recover them. You should only use this if doing a full revert to backup is not a viable option. Do also note that if you have refreshed the metadata on any tv series since the backup, it could cause issues with the episode posters.

#Run the following commands as root
sudo su

#User variables
SERVICE_NAME='plexmediaserver'
BACKUP_LOCATION='/ROOT_BACKUP_LOCATION/'
PLEX_USER='plex'

#Derived variables (Slashes at end of directory paths are required)
SQLITE3_PATH="/usr/lib/$SERVICE_NAME/Plex SQLite"
DB_PATH="/var/lib/$SERVICE_NAME/Library/Application Support/Plex Media Server/Plug-in Support/Databases/com.plexapp.plugins.library.db"
IMG_PATH="/var/lib/$SERVICE_NAME/Library/Application Support/Plex Media Server/Metadata/"
CACHE_PATH="/var/lib/$SERVICE_NAME/Library/Application Support/Plex Media Server/Cache/PhotoTranscoder/"

#Stop the plex service until we have updated everything
echo "Stopping Plex"
systemctl stop "$SERVICE_NAME"

#Set the (user_thumb_url, user_art_url, and updated_at) from the old database in the new database.
#The updated_at is required to match the cached images
echo "Updating database"
sudo -u "$PLEX_USER" "$SQLITE3_PATH" "$DB_PATH" -cmd "ATTACH DATABASE '$BACKUP_LOCATION$DB_PATH' as OLDDB" "
UPDATE
  metadata_items AS NMDI
SET
  user_thumb_url=OMDI.user_thumb_url,
  user_art_url=OMDI.user_art_url,
  updated_at=OMDI.updated_at
FROM
	OLDDB.metadata_items AS OMDI
WHERE
	NMDI.id=OMDI.id
"

#Copy the posters and art from the old directory to the new directory
echo "Copying posters"
cd "$BACKUP_LOCATION$IMG_PATH"
find -type d \( -name "art" -o -name "posters" \) | xargs -n100 bash -c 'rsync -a --relative "$@" "'"$IMG_PATH"'"' _

#Copy the cached images over
echo "Copying cache"
rsync -a "$BACKUP_LOCATION$CACHE_PATH" "$CACHE_PATH"

#Restart plex
echo "Starting Plex"
systemctl start "$SERVICE_NAME"

The below commands are tailored for Linux, as file paths are different for Windows. They need to be ran with the sudo -u plex prefix for proper user access permissions.

'/usr/lib/plexmediaserver/Plex Media Scanner' --list

'/usr/lib/plexmediaserver/Plex Media Scanner' --analyze --manual --server-action intros --section LIBRARY_ID

'/usr/lib/plexmediaserver/Plex Media Scanner' --analyze --manual --server-action credits --section LIBRARY_ID

sqlite3 '/var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Plug-in Support/Databases/com.plexapp.plugins.library.db' '
SELECT MDI.id, file, title, MDI."index", hints, GROUP_CONCAT(T.text) AS taggings
FROM media_parts AS MP
INNER JOIN media_items AS MI ON MI.id=MP.media_item_id
INNER JOIN metadata_items AS MDI ON MDI.id=MI.metadata_item_id
LEFT JOIN taggings as T ON T.metadata_item_id=MDI.id AND T.text IN ("credits", "intro")
GROUP BY MDI.id'

Yesterday I moved my Plex installation from a Windows machine to a Linux machine. The primary data folders that needed to be copied over were Media, Metadata, Plug-ins, and Plug-in Support. It doesn't hurt to copy over some of the folders in Cache too. It's possible there may be some more data that needs to be moved over, but I don't have documented what.

After moving all the data, I updated the paths in the database for the new machine. Doing this allowed me to keep everything as it was and no new refreshes/scans needed to be done.

The location of the Plex modified SQLite for me was at /usr/lib/plexmediaserver/Plex SQLite. So the following is the bash commands to stop the plex server and open SQLite editor on Linux Mint 21.3.

service plexmediaserver stop
/usr/lib/plexmediaserver/Plex\ SQLite '/var/lib/plexmediaserver/Library/Application Support/Plex Media Server/Plug-in Support/Databases/com.plexapp.plugins.library.db'

And the following is the SQL I used to replace a D: drive with a path of /home/plex/drives/d/. You can replace these strings in the below code with your custom drives and paths.

#Replace backslashes with forward slash in paths
UPDATE media_parts SET file=REPLACE(file, '\', '/');
UPDATE section_locations SET root_path=REPLACE(root_path, '\', '/');
UPDATE media_streams SET url=REPLACE(url, '\', '/') WHERE url LIKE 'file://%';

#Replace root paths
UPDATE media_parts SET file=REPLACE(file, 'D:/', '/home/plex/drives/d/');
UPDATE section_locations SET root_path=REPLACE(root_path, 'D:/', '/home/plex/drives/d/');
UPDATE media_streams SET url=REPLACE(url, 'file://D:/', 'file:///home/plex/drives/d/') WHERE url LIKE 'file://%';
UPDATE media_streams SET url=REPLACE(url, 'file:///D:/', 'file:///home/plex/drives/d/') WHERE url LIKE 'file://%';
UPDATE metadata_items SET guid=REPLACE(guid, 'file:///D:/', 'file:///home/plex/drives/d/');