[CentOS] nightly rsync has started to throw errors

Hi all,

I perform a nightly snapshot of /home to a USB attached drive scheduled
via cron.  The system is CentOS 5.2 and only gets attached to the
internet periodically for updates, otherwise serves as a samba server to
about 20 Windows clients.

The rsync command being used is:
rsync -av --delete /home/ /media/bkup320G/
and has been working well until a few days ago.

Starting with a few days ago, my nightly rsync/cron emails included some
errors as shown here:

rsync: mkstemp
"/media/bkup320G/cprcvs/c/Projects/WindowApps/DLL/HMRControlDLL/.CMDSettingsDialog.cpp,v.I5QnaM"
failed: Read-only file system (30)
rsync: failed to set times on
"/media/bkup320G/cprcvs/c/Projects/WindowApps/DLL/PSIControlDLL":
Read-only file system (30)

There are multiple issuances of these errors for different files....some
files are written just fine.  Where there are errors, its always the
same Read-only file system (30).

Output of df and mount show all filesystems as (rw):
# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                     136838136   5518276 124256720   5% /
/dev/sdc1            283783548 167456104 101679380  63% /home
/dev/sdd1            283783548   4648856 264486628   2% /home/905
/dev/sda1               101086     31745     64122  34% /boot
tmpfs                   452084         0    452084   0% /dev/shm
/dev/sdg1            307663800 171859328 120176040  59% /media/bkup320G


# mount
/dev/mapper/VolGroup00-LogVol00 on / type ext3 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
/dev/sdc1 on /home type ext3 (rw)
/dev/sdd1 on /home/905 type ext3 (rw)
/dev/sda1 on /boot type ext3 (rw)
tmpfs on /dev/shm type tmpfs (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
/dev/sdg1 on /media/bkup320G type ext3 (rw)

Googling has not been fruitful in this issue and I'm curious what might
be the issue.  I'd originally thought it might be drive failure on the
external (destination) drive, but changing that produced the same errors.

We typically backup about 165G overnight, so initial write to a new
drive takes a LONG time.

Any pointers or suggestions would be greatly appreciated.

TIA,
-Ray