I need a backup to USB script with rotating drives

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http://www.linuxquestions.org – I have a bash script that uses rsync to backup data on a daily basis to a USB hard disk and it's been working fine because the USB drive is always connected, however, the volume of data we need to backup has grown and I've had to split it across 4 x 2TB drives set as 2 spanned drives internally. The first set is called FS1 and the second FS2, which I mount into the following two folders and make them available to our network /srv/FS1 /srv/FS2 Both have 4TB capacity. What I am trying to achieve is to create an automated backup script that runs from a cron job which checks what USB drive is a (HowTos)