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external drive is only good if you have a thunderbolt or ESATA connection, and even then you will never be faster than an ESATA 6gb/s connection directly into the MOBO, unless you have a raid array in thunderbolt, which is pretty costly. USB 3 is fast, but never reaches advertised speeds and bottlenecks pretty quickly.
Multiple SSD's for the cache, catalog and OS/Programs are a better route, with your files on some sort of raid array with spinning disks. I personally have a raid 5 array of 8 2tb HDD's for my working/files drive that are connected to an Areca raid card which gives me 850 MBS read and 700 MBS write and is blazing fast, much faster than you'd ever need for LR to utilize Always, and I mean always have backups of your working files - raid is not a backup solution, unless you are talking enterprise level, which we are not.