How to Fix Mac Failing to Mount External Hard Drives?

According to your description, I can rule out the error on your external hard drives and your iPhone. Instead, I suggest you perform a minor macOS update, from macOS 12.1 version to macOS 12.3 version, which is able to solve your trouble without effort.

Here is how to update your macOS 12.1 to macOS 12.3:

Step 1: Click the Apple menu on the upper left corner of your desktop.

Step 2: Select About This Mac from the dropdown menu and then head to click Software Update in the main window.

Step 3: You can see the available macOS 12.3 update listing on the window and click Restart Now to begin the macOS update.

After the upgrade, head to see if your Mac mounts the external hard drives and iPhone successfully.