Take a look at the total size of the /opt/zimbra/backup folder. With the current method Zimbra uses all backups are stored there with minimal compression for speed. For example, I have roughly 300gb mail and files in my store folder, but 800gb in backups. To solve this I added a 1tb RAID with larger slower SATA drives, rather than faster SAS, and mounted that RAID at the /opt/zimbra/backup. It was simple and also allowed me to use the HSM function as well.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now