Long story short, I have had to resort to a laptop to do my Unity work for the time being with the Unity editor being run off an external hard drive connected via USB 3.0.
Short of the restrictions placed by my actual processing hardware, how much of a performance hit am I taking by streaming everything from the external drive as opposed to running it from my main HDD?
(yes, I am literally storing and running everything Unity related from this drive. My local HDD is pretty small in size but I could work on a small project locally if the performance hit was that significant)
I don’t see what this has to do with Unity.
SATA-II (assuming that’s what your internal HDD uses) has a max transfer rate is 3.0Gb/s. USB3.0 is 5.0Gb/s. You could find this out yourself using Google.
So you have no performance “hit” - you have a gain in maximum bandwidth. But it’s largely irrelevant because your HDD itself probably won’t have data read/write rates anywhere near that high anyway (though you haven’t provided enough details to know).
And in the end that’s all theoretical anyway - you’ve got the hard drive in front of you - try running Unity from it. Does the performance seem acceptable?