An external disk refuses to mount, com.apple.DiskManagement.disenter error -119930868.

Sadly, First Aid didn't do much, as the problem was specific to one machine. I ended up splitting up the group of disks and backing up my data using a third party program called Sync Folders Pro. As I did the backup, I noticed that the drives were very slow. Checking on the speed, I discovered that they were operating at only 30 Megabytes a second. Normally, they work at 140, so this is a significant slowdown. I checked the hub they were plugged into, by speed testing an SSD, and that was fine. In the end, I had to do the backup one drive at a time, with the backup and source drives plugged directly into the machine and swapping them as required. That was the only way to get them back to full speed and have the backup take 2 days, instead of more than a week. The reason for the slowdown and the drives refusing to work together remains unknown, but with only having 2 USB ports on my Mac Mini (and 4 Thunderbolt ports), there is no way I can plug all the drives in directly. With so few ports, they are needed for other essential devices (like my eGPU).

On the good side, the backup was completed, even if it wasn't via Time Machine, and I have a NAS due to arrive tomorrow. To me, a NAS is the only real solution to the ongoing issues with hard drives that macOS has (lack of ports, random disconnections, slowdowns, disappearing drives [that are mounted but don't show up on the desktop], random disk errors, permission problems, etc.)

Tried that, didn't work. Couldn't even reformat the latest drive using using Disk Utility. It refused to do it. Plugged the drive into a Windows machine and reformatted it there to NTFS. Then plugged it back into the Mac and Disk Utility was able to do its job. Then recovered the drive (it was full of cloud data, so nothing lost). Not the first time I've had to do something like this, as this SSD was a replacement for an earlier drive that was rendered irrecoverable by the same bugs. I'm just glad the new drive is still usable, but I'm well aware that all external drives are now at risk and can be rendered inaccessible at any time. Have already transferred as much data as possible off external drives and onto my NAS. Drives with data that is not used every day, I have unplugged from my machine so the OS can't affect them.

Open Activity Monitor and type in "fsck." If a process is shown, force quit it, the drive should now show up in Finder.

How are/were the drives formatted? If they were NTFS then the upgrade removed the software required for that format and will have to be reinstalled.

I could not find the fsck process in the Activity Monitor. I purchased the Tuxera app. It works for me 100% of the time.

What worked for me was relaunching the finder (Click Apple top left -> Force Quit... -> Finder -> Click Relaunch)

After that, the drive was automatically mounted. Hope this helps someone!

correction: it was sophos!! If you use sophos antivirus, that might be the problem!

1. Go To Disk Utility
2. Select External Drive
3. Press “Erase”
4. Name the Drive
5. Format as MacOS Extended (Journaled)
6. Leave “Scheme” as is.
7. Click Erase 

I was able to solve the problem by entering the activity monitor and forcing the finder to close

It really help me. Do no remove hard disk directly. If you directly remove then mac will unmount the hard disk for system. Goto disk utility and click mount or first aid option.

This worked for me also, I found I had to do this every time I used the drive.

I left the fsck to run its course (about 30 minutes for 1TB) and the disk works correctly every time now.

That worked for me. Opened Activity monitor and force quit fsck_ext. Thank you so much!!

Hey all,

I found a fix for this.

update dyld shared cache -force

reboot in safe mode

reboot again normally

It worked of 4 computers for me so far

Let me know if it works for you

I am not agreeing with wrongly ejected position. This clearly a kext file disrupting the usb mount. If you boot in safe boots it will mount. The big question is why and which I extension is it.

In my case, it took a lot of time. I finally ended up letting my external drive run all night and it was mounted in the morning. I don't know how long it actually took to check a 2TB drive, but after ejecting it and connecting it to a new iMac with Catalina, Migration Assistant is doing its thing. Patience is the key.