Disk not ejected properly

That external drive could have a bad cable, a bad chip in the enclosure or the drive is failing. Which in-turn, can take down similarly connected drives and devices on the same Bus.

You didn't answer what external drives you have or how they are connected to the Mac?

We might be able to better help you with a bad external drive or troubleshoot and fix it, if we knew what you had.

This is getting really frustrating for me, too and I'm seeing people with this problem everywhere. I have tried two LaCie's, a G-Technology (brand new), and a couple of Seagates. I've tried all four of my thunderbolt ports. Every time my MacBook Pro 16 goes to sleep, I get this message when I wake it up. It's not the cable or the drive, folks. Something is going on here

Yes! This has been happening to me ever since I received my new Macbook Pro 13" w. four Thunderbolt ports (in late August). I have 2 Seagate USB external drives w. 2 partitions each; one is half Time Machine & the other is half SuperDuper clone. Every day when I sit at the desk I'm greeted with four Disk Not Ejected Properly messages. I have swapped the HDs' locations in the chain (I have two Thunderbolt docks & and a Thunderbolt to USB adapter) there is no change. Is it the problem in Catalina or the Macbook?

System Preferences > Energy Saver > “Uncheck” - Put hard disks to sleep when possible

What external hard drives and how are the drives connected to your new iMac?

Keep Drive Spinning...

http://jon.stovell.info/software/