Postgres initialization of pg_basebackup
A physical backup (like pg_basebackup) results in an already initialized directory.  You don't run initdb.  You just start it up, possibly after changing the config files in whatever way is appropriate for your situation. 

You probably should have configured this for streaming though with a named slot though, or it will have fallen behind while the backup was being taken, but that would be a different question.

Why not just move the drive array from one server to the other?  Is the desire to change the storage hardware as well as the server hardware?