Union mount support was added in version 3.8.1 through UnionFS. This was later replaced by Aufs in 5.1.0 to improve stability. The union mount allowed virtual updates to the data on the read-only CD/DVD media by storing changes on separate writable media and then representing the combination of the two as single storage device. The writable media could be memory (ramdisk), a hard disk, USB flash drive, etc. This means that the user could modify the software installed on the Knoppix system, such as by using APT to install or update software. The storage device containing the changes needed to be present whenever Knoppix is started, else only the original data from the disc would be used. While Knoppix would scan available storage devices for a persistent home directory by default, a user could dictate a specific location with a boot option (see below) such as: