Hi Ric,
Sadly you cannot just connect another HD via the USB to give you more space.
You either need to replace the internal HD with a new one or back up your existing HD to CDs or to a PC (via USB). Remebering that you have alot of data to back up your PC's HD will need to be big enough to cope with it all. Backing up songs is very simple, but time consuming no matter which method you choose. Changing to a new HD is also very easy and there are instructions floating about (hopefully on this forum)
Once you have backed everything you want to keep then you just delete / reformat the HD in the 2488.
You could also try using the Delete Unused function to clear out data from songs. This will give you a little extra space but is not a long term solution to filling up up the HD
Lastly, as to future proofing - I now make a back up of the .wav files of the tracks I use when mixing down. I do this because in the past I made only data backups from my ole 788. I then got a 2488 and sold the 788 - now I have data backups that are useless unless I can get a 788 to restore the files too.
HTH
Zeek