Sometimes I see the Linux kernel being mentioned in the list of upgrades, when running pacman -Syu (updating my packages in Arch Linux). Whenever this happens, after installation of the packages, I can not mount USB drives anymore until I restart. I would just like to know if this is something that is common and expected (and if so, why, do I wonder), or if this is something not expected that I should investigate. 
Is it normal that a restart is required to mount USB after a kernel upgrade?
kernel;usb;upgrade
Probably, on that distribution, it is normal. It depends on how the package manager installs the new kernel.I suppose that your package manager (when upgrading the kernel) deletes the old kernel-modules directory immediately. This way, when you try to mount a vfat-formatted usb stick, the kernel will fail to load the needed vfat kernel module.To verify my supposition, next time you upgrade the kernel, you can check the existence of kernel module directory:before the upgrade, you should find that it exists a directory named as the current (the old) kernel version.~> ls -d /lib/modules/`uname -r`/lib/modules/3.0.0-1.2-desktopafter the upgrade but before the reboot, you should find that the directory does not exist any more (so you cannot manage new hardware).~> ls -d /lib/modules/`uname -r`ls: cannot access /lib/modules/3.0.0-1.2-desktop: No such file or directoryafter the reboot, you should find that it exist a new kernel module directory named as the current (the new) kernel version. ~> ls -d /lib/modules/`uname -r`/lib/modules/3.1.0-1.4To avoid this problem, other distributions (like openSuSE) delay the directory deletion till you reboot.