ok..
the reason your ipod keeps loosing music is because it is set to sync with your itunes software.
meaning... if the ipod does not find the same music on itunes then itunes will delete the music off of the ipod.
hence why you are experiencing music missing off your ipod.
i do not believe that all is lost. it should still be possible to salvage your music.. because in essence no file is absolutely deleted unless you do a secure wipe of the drive (let that be a warning that other people can view your deleted files if they have the knowledge).
try connecting the external harddrive (a.k.a HDD) to your laptop. once everything has been connected press the start button in the bottom left corner then press the search function. try and inputting one of the songs that you know was part of your library but went missing.. make sure to click on the "pictures, music, video" option to help narrow your search and reduce run-time.
if you find the songs you will most likely be able to find all the others. once you find the files you can right click the music file then press "open containing folder" this should open a new window which is the location of the music files.
make sure to note this location as it will be where you can find your music.
after that has been completed and you know for sure where the files are, delete the contents of your itunes library. DO NOT delete the actual files, itunes should prompt you whether or not you want to delete the files, DO NOT DELETE THE FILES.. just delete off of itunes (removing off of the list).
now.. take all the new audio files which you have found and then add then to your itunes, please keep in mind that from now on you will have to have the external harddrive connected to your laptop prior to turning on itunes or syncing with your ipod.
hope this helps.
feel free to post again if you need more help.
cheers