We currently are using Arcserve to do disk-to-disk-to-tape backups. We have a Drobo Elite with 10 TB of storage as the target for the "to-disk" part of the backup. The Arcserve server (Dell PowerEdge r420 running Win 2008) is connected to the Drobo via iSCSI (Gigabit).
The throughput for this, according to the Arcserve logs is around 250 - 500 MB/min, depending on the server/OS; Win 2003 and old hardware = very slow, Win 2008 and newer hardware not as slow.
For comparison, I setup a test backup that used disk space on the Arcserve server itself (RAID 10 SATA-600). Now the log speeds are 850 MB/min - 2.37 GB/min.
Internal server storage is high performance and RAID protected so it's relatively expensive and best used for daily user usage.
I'm looking for recommendations on external storage (Approx 6 TB) that can run at near SATA-600 speed and support RAID 5, 6 or 10 and not break the bank. I want to keep about 4 weeks of "disk-to-disk" data. Restores of files from disk are fast and convenient and 4 weeks gives me some opportunity to restore previous versions of files.