Upgraded to Monterey a month ago and started seeing this issue with my G-Technology external disk used for Time Machine (I've been using that for over 4 years w/o problem). Extremely frustrating and quite scary to contemplate losing years of backups. Changing the usb cable and power supply to the external disk didn't solve the problem so finally had to buy a used 5th Gen Apple Time Capsule to use for backups. That's thankfully working fine but Apple really need to resolve this bug. With every major MacOS upgrade, Apple take a few steps forward but break critical features in the process.