lbu with ro-mounted media and "keep X backups" does not work

lbu commit

mv: can't rename '/media/usb/asterisk.apkovl.tar.gz': Read-only file system
* Saving PostgreSQL databases to /var/lib/postgresql/backup/databases.pgdum [ok]
cp: can't create '/media/usb/asterisk.apkovl.tar.gz.new': Read-only file system
mv: overwrite '/media/usb/asterisk.apkovl.tar.gz'? y
mv: can't rename '/media/usb/asterisk.20110323091154.tar.gz': No such file or directory
Problems creating archive. aborting

(from redmine: issue id 562, created on 2011-03-29, closed on 2011-04-27)

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