You mentioned “plugged in my external HDD”. I presume you are using a USB drive. When it connects to RPi, the contents are mounted somewhere which can be accessed by applications. This “somewhere” most likely got changed when you last unplugged and plugged it back.
Try to locate this mount folder (perhaps called “mnt”) then see if you reference the same path inside Jellyfin.
You mentioned “plugged in my external HDD”. I presume you are using a USB drive. When it connects to RPi, the contents are mounted somewhere which can be accessed by applications. This “somewhere” most likely got changed when you last unplugged and plugged it back.
Try to locate this mount folder (perhaps called “mnt”) then see if you reference the same path inside Jellyfin.