How to install SSD on late 2007 MacBook Pro (15 inch) I have a slot on the left-hand side of my MacBook Pro (late 2007, NOT a unibody) and am wondering: is this slot suited for an SSD that I can simply stick in? Or would I have to install an SSD internally?
Since Yosemite needs drivers for those and it has to be signed(here is discussion of possible problems with that kind of SSD on Apple discussions) you much better off installing SSD inside the MBP in place of HDD.

Here is instruction from iFixit how to do it.