You can define file systems using the fileSystems configuration option. For instance, the following definition causes NixOS to mount the Ext4 file system on device /dev/disk/by-label/data onto the mount point /data: fileSystems."/data" = { device = "/dev/disk/by-label/data"; fsType = "ext4"; }; Mount points are created automatically if they don’t already exist. For device, it’s best to use the topology-independent device aliases in /dev/disk/by-label and /dev/disk/by-uuid, as these don’t change if the topology changes (e.g. if a disk is moved to another IDE controller). You can usually omit the file system type (fsType), since mount can usually detect the type and load the necessary kernel module automatically. However, if the file system is needed at early boot (in the initial ramdisk) and is not ext2, ext3 or ext4, then it’s best to specify fsType to ensure that the kernel module is available.