macos – “DISK NOT EJECTED PROPERLY” every 60 seconds or so

I attached a 2T magnetic, spinning USB disk to my mac mini today, and started getting:

DISK NOT EJECTED PROPERLY
Eject "Untitled" before disconnecting or turning it off.

But I’m not ejecting it, or removing it, and I’m not futzing with the cabling.

It happens pretty precisely every 60 seconds. It’s a pain to keep dismissing all the popups.

I do not have energy saver set up on the drive in macOS. And I am currently running a backup to it. so it’s probably not idle.

This hardware worked fine on a Linux system.

I’m using macOS 11.5.2.

Any suggestions?

Thanks!