On a fresh install, Mojave seems to go a little nuts with Spotlight indexing. That means a tremendous amount of disk access, possibly even while your portable is "sleeping". There's a lot of disk I/O going on in Mojave behind the scenes, and I have some reason to think it doesn't handle corrupt files particularly well, so that could be contributing to your problem. Also, Mojave defrags in the background. Previous releases did as well, but combine that with its apparent difficulty accessing files it thinks are corrupted, and you might end up with the drive being hit even more than usually.
So best bet, for a portable, is to leave it plugged in for a day or two, at all times, after install.
(I put up a fairly lengthy discussion here; some of the issues I've found may be related to what you're experiencing: Frequent system hangs and freezes in macOS Mojave 10.14: Notes, reproducible errors, and possible workarounds)