Your computer is going to have a hard time playing HEVC and H.264 on the fly. It requires signifigant processing power to do so. Additionally, a Toshiba Canvio is not going to cut it for multiple streams of HAP video playback.
If you run the black magic disk speed test on the Toshiba with UBS 3.0, what are you getting MB read and write? Maybe somewhere around 80-90MB/s? A 4K HAP video may require anywhere from 47-1,000MB/s read speeds depending on the data and framerate. So if you’re trying to play 2+ 1080p60 HAP videos off your external drive, the Toshiba is not going to cut it.
Since the file size of H.264 and HEVC are smaller, it won’t be a HDD bandwidth issue, but will move the stress on your machine over to the CPU, limiting the number of FX, plugins, and other tools that you can use in your setup.
First, I would shoot for a drive with 200+MB/s read and write. Seagate used to make a USB 3.0 Fast drive, 300MB/s read write, up to 4TB, but the discontinued these and now they are only solid in the more expensive Lacie Rugged Raid $250.
At that point, you are better off going for an SSD (IMO). I started with 2x 1TB SSDs and would swap out what I needed per gig. Now I have a 4TB SSD with a 4TB HDD backup that I keep everything on. Also, with a newer machine 2019+ H.264 & HEVC decoding has gotten better, so now I can run all H.264 (with a keyframe for every frame) without any issues.
If you don’t have any other choice. Make sure that every single H.264 file has a keyframe for every frame. Also refered to as group of pictures (GOP). This indicates the space between i frames and p frames. The more compressed an H.264 is, the more CPU power is required to decode it on the fly – slowing down your system and lowering playback performance.
It took me some years to figure all this out, but if you find the right configuration for your setup (don’t update), you should be able to VJ off that machine for years to come. I ran all HAP videos for my 2015 MBP for 7 years without issues.