This will work but as stated by ss202sl it will likley lead to problems. Your SSD is likley large enough to load all of your porgrams on. Then just use the external HDD for data storage. Save for an internal HDD. If you want to install programs to the external HDD be prepaired for it to be slow unless you are using a USB 3.0 controller and a 3.0 Drive. 2.0 will max out at about 25MBps which is about the speed of an HDD from 1997.