I recommend that the external drive is, at minimum, a blue plug being fitted into a blue USB socket because this gives you most of the benefit that you really want from an SSD (which is fast load times).

SSDs suffer from a problem which is that each individual cylinder can run out of read/write cycles. Once this happens, the file using that cylinder becomes corrupted. This means your game assets will randomly become corrupted and cause your game to crash eventually in different ways/situations.

This never happened to me with a HDD, but with an SSD it has already been a problem for me multiple times. If you copy+paste each individual file in the game directory, it eventually will fail to copy the ones that are corrupted. In many cases the only way to properly get the files fix anyway is to fully redownload the game because SE doesn't seem to have a simple scan and repair system that saves time and bandwidth. This can also happen just to your configuration files in Documents\My Games.