I've been having the same problem with Mjoave 10.14.6 for AGES now on my 2009 Mac Pro. Directly related problem, in the middle of using the machine, on, working on anything at all, a drive will eject. And I mean it doesn't even show up in Disk Utility in Mac OS, or the one in terminal. I have to reboot the computer for the disk to show back up. The disk is formatted as ExFAT so I can access the files on my windows & linux machines. But I've had the same thing happen to a disk that was formatted jhfs+. And it ALWAYS happens to ANY external drive I have connected, if the computer goes/is put to sleep. Upon waking, it tells me that every external volume was ejected improperly. I've never had this problem before. Rebooting every time this happens is INSANE. Yes, I know having to do it if you leave the machine overnight, but if I'm directly in the middle of something? Awful! Oh, a little PS: One time this happened to the drive, tried running first aid on it, on the disk, it said it was fine, on the volume, it failed. Because it was ExFAT, I pulled the drive, put it in my windows 11 machine, and ran the default check disk on it, and it took a while (the disk is 8TB) and fixed it and was fine. Put it back in my mac? Failed a few hours later that day. My Mac Pro is Mojave 10.14.6 (checked before posting this, and frequently - no updates available) Early 2009 2x 3.46GHz 6core Intel Xeons 48GB 1333MHz DDR3. The system boot disk is a sandisk 1TB SSD