I posted my issue on a separate post the other day-but received little help. So since I have almost the same issue as the initial poster, thought I'd post here instead. I've owned Macbooks and iMacs and I have never had issues like I have had since upgrading my 2017 MBP to Mojave this past Dec. My problems began when I upgraded to Mojave (from the latest version of HS)- I keep things updated very well. I initially noticed that it would freeze and the only way to fix the issue was a power off. Then I noticed that my External HD (Mac OS Journaled Ext) that I have only had for a few years would take forever to load up and eventually show my files. Took up to 10 minutes sometimes. So I decided to do a fresh install of Mojave. Did a TM backup and a CCC backup. Reinstalled Mojave as a NEW Macbook Pro without loading my backup. Had some issues with FileVault, so I wiped and reloaded it again for the 2nd time. I worked on it a bit and it seemed fine so I slowly loaded some apps little by little before advancing too much further. Then I eventually plugged my external hd in. Same thing happened-except now it got to the point it would not even show up in Finder. Bought a new USB cord-not the issue. So I rushed out and purchased a new WD external HD with USB-C (formatted it APFS instead of Mac OS Journaled after reading it was best to use with Media/Images and with Mojave). It was awful trying to move files over when the other EHD wouldn't show up in finder but after a week, I eventually got them all moved over to the new external hard drive by plugging it into my NAS Drive on my network. So when I had no issues with this new EHD, I assumed it was my old external hard drive with an issue that just coincided with the Mojave update. Especially since a couple of my other older EHD worked fine with Mojave from the get go. Anyway, I have been using the new drive now for about 4-5 weeks and have had no issues until last week when all the sudden I notice that files are missing that were there! Sound familiar? So after a few hours, I plugged the new USB C HD in again-and NOW it won't show up either. Again, older EHD's will show, but this brand new one won't all the sudden just like my other EHD did. It shows in Disk Utility but doesn't show how much of the EHD has been used. I tried safe mode, reset SMC, PRAM - didn't fix it. So I now have 2 external hard drives (formatted differently) that have been bricked by Mojave. Took it to Apple and they eventually decided that nothing was on the external hard drive and I disagreed. They wanted to wipe it for me-I said no. They suggested that APFS formatting on be done on SATA Hard drives, not external hard drives because they do not play well with Mojave and APFS. WHAT? Anyway, my first External that quit showing up was formatted Mac OS Journal Extended. So I went back home and purchased Boysoft data recovery software and it has found all my files so far. So I am trying to recover the files that I had added in the last few weeks (all others were backed up thankfully). And it gets better - the first external hard drive that stopped showing up on Mojave back in December that started all of this, now shows up perfectly fine in Mojave. WHAT?
So my concern is that this has now happened to me twice on external hard drives - why? And yesterday, I had another external that stopped showing up in Finder, but would show on my desktop. Praying it isn't doing the same thing. Any suggestions or any info that has been discovered since this initial post?