Does anyone know how feasible this is? I'm also concerned about the cost. I'd also like to be able to partition the external drive in order to use it to install both Mac programs and Windows programs.

It's essential that the programs don't suffer any significant performance loss while being run from the external, if I go that route. I'd imagine it would have to use either Thunderbolt or USB 3.0 in order to be fast enough, and it may be better if it's a solid state drive, but I'm not sure. I've heard that a fast enough external drive can function as well as an internal drive for running software. So, I've been considering two solutions: either I try to upgrade the internal storage, or I try to find an external hard drive fast enough to install and run large, cpu-intensive programs. I have a (slow, USB 2.0) external hard drive where I can store files, but there are several large programs that need to be installed on the internal drive, and I don't have room for them all. I've soon come to realize that the remaining hard drive space on both partitions is insufficient for my needs. There are programs I need to run on both Mac and Windows, so I've partitioned my hard drive using Boot Camp. I have a 2013 Mac Pro that came with a 250GB SSD.

External ssd for 2013 mac pro