I use Pop!_OS 22.04 and today I encountered a nasty surprise: I couldn’t mount the portable USB disk that I use to back up my Bitwarden vault, which contains passwords and secure notes.1 Fortunately, it only took a few seconds of web searching to find someone who encountered the same problem.

Solution

Reboot, then mounting the USB disks worked as expected.

Why?

During an operating system package upgrade, I evidently upgraded the Linux kernel from version 5.x to version 6.x. This seems to have caused the usb_storage kernel module to stop working. This meant that I couldn’t mount any external USB disk, no matter which port (USB-C or USB-A). I infer that this kernel module changed in a non-backwards-compatible way and reloading it seems to have solved the problem.

I did not try reloading the kernel module with modprobe, so maybe that would have worked.

How Did We Get Here?

I connected a USB disk (call this “Disk A”) to my System76 Lemur Pro’s USB-C port. It did not mount.

I connected Disk A into my Galaxy Tab. It mounted. I inferred that Disk A works.

I found another USB disk (call this “Disk B”) and connected it to one of my Lemur Pro’s USB-A ports. It did not mount. I inferred that this is a software problem instead of a hardware problem, because what are the chances that both ports are dead at once?

I searched the web: lemur pro usb-c port doesn't work. I found a comment suggesting to check dmesg for related log messages.

$ sudo dmesg | rga -i "usb"
[...a bunch of stuff...]
failed to validate module [usb_storage] BTF: -22
[...a bunch of stuff...]

That seemed like a sign of trouble, so I searched the web: "failed to validate module" usb_storage. I found https://forum.proxmox.com/threads/usb-storage-can-not-be-mounted.100480/. This is how I learned about journalctl -f for monitoring the system logs in real time. I connnected and disconnected the USB disks and watched the messages in real time, confirming the symptom I’d seen. The person who asked the question in this thread reported

I tested again yesterday after a restart. Now everything is working as expected.

I didn’t love trying a magic solution, but I happened to use the command uname -r and noticed that I’d upgraded the kernel to a new major version, namely 6.0.x. That gave me the idea that maybe a kernel module had breaking changes and needed to be reloaded, then suddenly “reboot” didn’t sound like a magic solution any more, but rather made perfect sense to me.

I rebooted. Now all my USB disks mount as expected.

  1. Yes, I deleted my Lastpass account after the breach reported in late December 2022. Thank you for your concern. I hope my master password was strong enough.↩︎