Official comment
Hey Sebastian,
There was a security issue found with rekordbox versions prior to 6.2, so they have been removed from the archive. rekordbox 5 did not have this hole, so those versions remain available. You can install 6.2 and use it the same as any other 6.x version.
Provided you have access to the old drive and the contents of the Pioneer folder are available, if you put them into the same path for the new computer / drive (this requires the username to be identical on both old and new), it will work just fine.