webman's description would do it. The mv command would do it fine though without permission loss.
Shutdown Zimbra first before moving files or remounting. Also make a backup if you can.
View mounted drives and available space.
mount /dev/sdx# here /tempdir
mv -R /opt/zimbra/* /tempdir
mount /dev/sdx# /opt/zimbra
If all looks good and the data is where it should be, restart Zimbra at this point and all should be 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