I went to initialize my first backup and received the following error with no details

Unable to create backup directory: /opt/zimbra/backup/zextras/

My setup is that I mounted a vmfs volume from an NFS share as a disk to my Zimbra VM. I then partitioned and formatted the disk as ext3. Mounted it on /opt/zimbra/backup R/W.

The problem was the owner:group was root:root. So I ran the following command as root

chown zimbra:zimbra /opt/zimbra/backup

Now it can write and is happy.
Thought I would share for the next guy.

Thanks for the great product!

Stephen