Can't mount external drives OS Catalina

First try resetting the SMC, Resetting the SMC is sometimes necessary to restore normal lower level system functionality to your Mac, Particularly for power and hardware related troubles. Do this by first shutting down your Mac and then once fully shut down hold Shift, Control, Option and power button for 10 seconds (this fixed it for me). If your still experiencing problems try resetting the NVRAM (This will turn of your find my mac so make sure to re-enable it if you have it on) So again shut down your Mac, then as soon as you press the power button hold these four keys together:

Option, Command, P, and R

Keep holding these keys for about 20 seconds, during which your Mac might appear to restart. A start up sound should play and you can release the keys after the second start up sound. And then the Mac should restart normally. Then try mount the external disk again.

Hope this helps :)

here's a suggestion that has worked for me. Downgrade to a different Mac OS. I literally tried everything that many forums suggested but the Catalina was causing this issue. Apple again proved you should never upgrade to their Mac OS since they publish it with many flaws, this grew me so aggravated that I will never update the MacOS since it complicates my life so much. :D

Force External Mac Drive to Mount via Command Line in Terminal - Gemsbok. Look this up on You Tube. It works for me, but you have to try it more than once.

I have the same issue. My LaCie external drive won't mount. I thought maybe it was my adapter, since the HDD is USB 3.0. So today I bought a cord that's HDD to USB type C. That didn't solve the issue. So it's gotta be macOS Catalina.

I was using Mojave on an external SSD drive as the startup disk for a late 2012 iMac. No problems. I upgraded the drive to Catalina. Though I have the external drive selected as my startup drive, it instead boots to the old HDD drive in my iMac. I now have two external drives mounted on my desktop, one named iMac (which is the drive I have used to boot from) and a new drive named iMac-Data. I can choose to boot from the external SSD drive 10.15 in utilities, but it is not working.

Thank you in advance for anyone who might have suggestions.

My problem has been solved. I upgraded the HDD in my iMac to Catalina, too. I actually never made it through the entire process of being able to use it. The 'setting up your mac' screen stayed up for about 8 hours, just spinning. I finally just forced a shut down, restarted in recovery mode, and then chose the external SSD drive to boot from. It worked, and my iMac is running again with the external SSD drive as the start-up.

I have the exact same problem! All my External HDDs stopped working on MAC OS Catalina, canot even mount a flash drive! How am I supposed ot back up my data to downgrade if I cannot plug anything to this **** macbook. Uploading to cloud not an option due to size.

Same problem. My external HDD has continuously been connected to my MBP for over a year. Until Catalina was installed. Now cannot mount the drive - it just acts as if it's not there. Connected via USB-c, and drive light is blinking. All is fine except Catalina. Anyone, please help!

FWIW, when your Mac get "stuck" on the "Setting up your Mac" screen. Powering off and rebooting will "un-stick" it.

I have the same problem, after the update my external disk are not able to be mounted. Please help me.

How does one downgrade? I suppose I can google it but I didn’t know you could do this. I just might do it. Too many glitches that I’ve noticed.

This is beyond annoying. I have a one year old LaCie drive and it's not mounting either. So what am I supposed to do now? I'm not a hardware expert and usually cannot resolve these kinds of things.