BetterRed, that's brilliant! Thank you! That also makes sense since the computer I use the most is an MBA which 'only' has 128GB of space and since my library is large, it's best to put it on an external HDD.
I keep all of the books

They're small and storage is so cheap that it doesn't really warrant me removing it.
PS: Do you have any advice on how to setup Calibre lib on an external storage/NAS? Would that warrant a topic of its own?