A lot of the time I wish i could view what I have on my external when I'm away from home/when it's not plugged in. i imagine this wouldn't be that difficult to do, but I can't think of a way to do it.
Something like DiskCatalogMaker might work for you. It looks good and is popular.
I use DiskTracker with El Capitan and it works fine. I've paid for it and have been using it for many years. I will say that DiskCatalogMaker looks better.