I’ve re read your situation in the #1 perhaps you using sys-usb ? remember that when you using sys-usb, it’s passing the usb controller to sys-usb and that would umount your detached boot drive. That’s why i said you need to turn off auto start sys-usb when you want to upgrade kernel.
And for your situation now if you want to upgrade kernel,
- boot as normal.
- umount your detached boot.
- make sure it already removed.
- uninstall recently installed kernel, then remove everything on the /boot
rm -rf /boot/* - turn off auto start sys-usb then reboot.
- boot as normal, install new kernel then turn on auto start sys-usb and reboot (or you can reboot first to use newer kernel, remove old kernel, then turn on auto start sys-usb and reboot).
I really keep it clean, so i always switch to new kernel then remove old kernel.