External hard drive issue from Sonoma.

That can be caused by a loose or worn USB port in the Mac or a bad USB cable.

First I would test the external drive in a different Mac USB port.

Then I would test a new USB cable with that external drive.

I'm not totally ruling out the macOS as an issue. Just saying that we have a dozen external drives that get used with two different Mac's and we've never had a falsely ejected drive running the earlier or latest macOS versions.

I just updated to 14.7 after having issues with my external hard drives self-ejecting. There wouldn't even be a pop-up that said "your hard drives were improperly ejected". They would just be gone. One was a SanDisk 2tb and the other was a 12tb spinner. They both disappeared at the same time, crashing my Logic project. They were both connected directly to my M1 Mac Studio. If it happens again in 14.7, I plan on Time Machinin' it back to Catalina so I can get my work done.

Again, I'm not totally ruling out the macOS. Just saying, that I have not had this issue with any macOS version.

Your problem sounds more like a loose connection, a bad cable, a failing drive enclosure or a failing drive and not a problem with the macOS version.

In your case, I would go to the Apple > About This Mac > System Report > select USB in the Hardware section and look to see how much Current each of those external drives is using.

If both drives are connected to the 2 USB-A ports of the Mac Studio, then try moving one drive to a USB-C port by using a USB-C to USB-A adapter.