Arch: System freezes when coping large files from usb

Add the following two lines to /etc/sysctl.d/99-sysctl.conf (create if not exists)
vm.dirty_background_ratio=1 vm.dirty_ratio=2
vm.vfs_cache_pressure = 50

To apply use sudo sysctl -p /etc/sysctl.d/99-sysctl.conf or restart.