WARNING: pg_stop_backup still waiting for all required WAL segments to be archived (1920 seconds elapsed)
HINT: Check that your archive_command is executing properly. pg_stop_backup can be canceled safely, but the database backup will not be usable without all the WAL segments.
When looking at ps aux on the master, I see the following:
postgres 30930 0.0 0.0 98412 1632 ? Ss 15:59 0:02 postgres: archiver process failed on 0000000200000E1B000000A9
The file mentioned is the one that it was about to archive, when the standby server failed. Somehow it must still be trying to "catch up" from that file which of cause isn't there any more, since I had to remove those in order to get more space on the HDD.
So the archive_command is failing because it is trying to archive a file that no longer exists.
One way around this is to remove the .ready files from the pg_xlog/archive_status directory, which correspond to the WAL files you manually removed.
Another way would be to temporarily replace the archive_command with one that will report success even when the archiving fails, until the archiver gets paste this stretch. In fact you could just replace the command with 'true', so it reports success without even doing anything.