High io usage and server hang

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http://serverfault.com – I have 3 dedicated server all with software raid-1 sata, centos or cloudlinux and cpanel. To all of them the last 10 days or so during cpanel incremental backup server has high io usage and hangs out. No server is overloaded with many account's (10 accounts -60 accounts and 300 accounts average each server). To the one with 300accounts ( ex-6 hetzner) i added a 3rd hd for /backup didn't see any change. I could check for hardware failure but to have the same issue to 3 servers and be hardware failure i would hit jackpot Any ideas to search for the issue? The following pictures may can help (HowTos)