I'm seeing this with Big Sur 11.6.8 So far I've found that it happens consistently with two external USB spinning disks used for the Time Machine. Often one of the two will mount, Time Machine runs fine. This happens to my macbook pro after using an external monitor/mouse. (External keyboard is bluetooth) After disconnecting the monitor/keyboard/mouse, sometimes one of the two external drives will work, if I plug it into a USB port that was NOT used for the monitor/mouse. Consistently, both external disks work after a laptop power-off/on, continue to work with multiple unmount/disconnect cycles. The first time I use the external monitor/mouse, then disconnect them, plugin the external disks, one or both disk will not mount. This looks like a bug in how the USB ports are reset after use. Apple support recommended updating the operating system, a good answer. However, reading through all of these replies, it really looks like a kernel or driver use case bug.