What process are you using to write those backups to the external HD?
Recently I had a similar failure with one of the three shell scripts that I use. (http://www.sumware.net/robfm3/?p=27)
I tried changing permissions of the recipient folder to no avail. Instead I recreated the shell script, loaded it as a new schedule and deleted the old schedule.
That has worked. FMS 18.104.22.1689 on a MacPro 1,1 running OS X 10.6.8.
Sadly, the server's event log and the emailed notification cannot advise of the write failure.
Hopefully this experience may help,
Writing the backups from with the FMS Console. Just setting a different path. Has been working for some years. Suddenly stopped as the external disk permissions had "changed". Did an OS reinstall and now all is well.
Am also able to push offsite to DropBox - just Batchmod'd the Dropbox folder...
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