Attention fellow simmers, I need help !

Technically you can use a portable hard drive to install and play games, but it can impact performance depending on the specs of the hard drive. Some portable hdds won't load as fast as a normal internal computer hdd. The cache size of internal drives is usually bigger too, this means the drive can handle and load more data at one time (which is fairly important for games). 

I think SSD drive are more suitable for games if you want to go the external way, but I'm no expert, so please ask the technical question to a knowledgeable person before spending any money.