Rsync 'failed: Read-only file system (30)' error?

Trying to backup to an external EXT3 formatted USB drive.

Worked fine in the past ... suddenly I'm receiving this error?

Thanks for ANY input!!

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INCREMENTAL Backup started. Fri Jul 20 07:52:56 PDT 2012

Job: 001
Protocol: rsync
Source: [media]/Music
Destination: localhost::USB_HDD_1/media/Music


sending incremental file list
.DS_Store
SyncToy_60eca71e-438c-42ab-bfaa-a11c1b78212f.dat
rsync: writefd_unbuffered failed to write 4 bytes to socket [sender]: Connection reset by peer (104)
rsync: recv_generator: mkdir "/media/Music/.AppleDouble" (in USB_HDD_1) failed: Read-only file system (30)
*** Skipping any contents from this failed directory ***
rsync: connection unexpectedly closed (43 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(605) [sender=3.0.9]


Backup failed. Fri Jul 20 07:53:04 PDT 2012.

Reason for failure:
Error encountered copying data from source path /media/Music ==> localhost::USB_HDD_1/media/Music due to unknown reason. Please see log.