pg_basebackup: initiating base backup, waiting for checkpoint to complete
pg_basebackup: checkpoint completed
pg_basebackup: write-ahead log start point: 3A6/59000028 on timeline 1
pg_basebackup: starting background WAL receiver
pg_basebackup: created temporary replication slot "pg_basebackup_15532"
3067352123/3067352123 kB (100%), 0/1 tablespace (..._BASEBACKUP\20200304/base.tar.gz)
As you can see, I turned on --progress to see when my backup is done. However 100% is reached for many hours already and the kB count keeps on increasing. I can see that data is being written to disk and that base.tar.gz gets larger and larger. The initial number that pg_basebackup presented to be 100% was ~2.2 TB, which I assume is wrong, as the datadir is ~3.5TB.
There is no additional activity on the server, just me, running pg_basebackup.