failed archive command

I'm trying to setup wal archiving to a secondary computer on OSX 10.6.5 using postgres 9.0.3.

Here are my settings in postgresql.conf on the primary box:

wal_level = archive
archive_mode = on
max_wal_senders = 1
archive_command = '/usr/bin/scp -B /Volumes/DataDrive/data/%p postgres@172.20.204.55:/Volumes/DataDrive/wals_from_master/%f'

The problem is that I keep getting this message over and over again in the postgres log:

FATAL: archive command failed with exit code 255
DETAIL: The failed archive command was: /usr/bin/scp -B /Volumes/DataDrive/data/pg_xlog/000000010000007400000086 postgres@172.20.204.55:/Volumes/DataDrive/wals_from_master/000000010000007400000086
LOG: archiver process (PID 17771) exited with exit code 1
unknown user 502

The archive command works if I copy and paste it into the Terminal under the postgres user. Any pointers as to what I'm doing wrong? Thanks.