I’ve installed Arch Linux on the Raspberry Pi for the first time, and after getting the package manager pacman to update all of the software to the latest versions I’ve been getting the following error:

error: try running pacman-db-upgrade

When running the upgrade command this permissions error was returned:

ERROR: You must have correct permissions to upgrade the database.

After looking at several forum posts I discovered that the upgrade made my filesystem read-only. To fix this edit /boot/cmdline.txt and add the rw flag

nano /boot/cmdline.txt

selinux=0 plymouth.enable=0 smsc95xx.turbo_mode=N dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p6 rootfstype=ext4 elevator=noop rw rootwait

Your installation my have a different cmdline.txt file, but the important bit is the rw flag, usually placed after the root filesystem is declared.

For more information see this forum post: https://www.raspberrypi.org/forums/viewtopic.php?f=53&t=97657