Such drives are cheap and plentiful on the likes of Amazon and other shopping sites - you need to know what flavour of USB you have though - should be USB 2.0 even on an oldish laptop but USB 3.0 is faster. They are great for offloading stuff that you don't access often - or for backups. Try and pick one from a reputable name or at least a reputable supplier that has good ratings etc. If using for a backup then its wiser to encrypt it when you set it up... and if you get really nervous you should archive your backups !
Personally I run with external, encrypted thunderbolt drives and a time machine on all of my systems because life is complicated enough for anything else !
Personally I run with external, encrypted thunderbolt drives and a time machine on all of my systems because life is complicated enough for anything else !

confused* 
Comment