IO Error in rsync while doing everyday backup

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http://unix.stackexchange.com – I have a sample NAS server (QNAP TS-210), with very limited Linux on-board (although a bit reinforced with Optware/IPKG). I'm a Linux newbie. After digging through Internet I was able to write my own backup script, using rsync for syncing files between NAS hard drive and external USB drives, and with CRON for running this script periodically. Everything was fine until few days before. E-mails generated by backup script started containing information IO error encountered -- skipping file deletion for many resources (folders) being backed up. When I run the very same script via SSH I got a lot (HowTos)