More information as I'm looking into this myself. It appears I found the root of the problem but I'm unsure as how to fix it. It wants to mount the directory but it's not empty? I'm going to try rebooting and see if I get the same message.
From the GUI Bootlog:
Fri Jan 16 18:17:53 2015: Setting parameters of disc: (none).
Fri Jan 16 18:17:53 2015: Activating swap...done.
Fri Jan 16 18:17:53 2015: Checking root file system...fsck from util-linux 2.20.1
Fri Jan 16 18:17:53 2015: /dev/mapper/pve-root: clean, 69321/1818624 files, 2829973/7274496 blocks
Fri Jan 16 18:17:53 2015: done.
Fri Jan 16 18:17:53 2015: Loading kernel module fuse.
Fri Jan 16 18:17:53 2015: Cleaning up temporary files... /tmp.
Fri Jan 16 18:17:53 2015: Setting up LVM Volume Groups...done.
Fri Jan 16 18:17:53 2015: Activating lvm and md swap...done.
Fri Jan 16 18:17:56 2015: Checking file systems...fsck from util-linux 2.20.1
Fri Jan 16 18:17:56 2015: /dev/mapper/pve-data: clean, 20/3661824 files, 275899/14625792 blocks
Fri Jan 16 18:17:56 2015: /dev/sdd2: clean, 242/130560 files, 52715/522240 blocks
Fri Jan 16 18:17:56 2015: done.
Fri Jan 16 18:17:57 2015: Mounting local filesystems...done.
Fri Jan 16 18:17:57 2015: Activating swapfile swap...done.
Fri Jan 16 18:17:57 2015: Cleaning up temporary files....
Fri Jan 16 18:17:57 2015: Setting kernel variables ...done.
Fri Jan 16 18:17:57 2015: Configuring network interfaces...
Fri Jan 16 18:17:57 2015: Waiting for vmbr0 to get ready (MAXWAIT is 2 seconds).
Fri Jan 16 18:17:59 2015:
Fri Jan 16 18:17:59 2015: Waiting for vmbr10 to get ready (MAXWAIT is 2 seconds).
Fri Jan 16 18:17:59 2015:
Fri Jan 16 18:17:59 2015: Waiting for vmbr20 to get ready (MAXWAIT is 2 seconds).
Fri Jan 16 18:17:59 2015: done.
Fri Jan 16 18:17:59 2015: Starting rpcbind daemon....
Fri Jan 16 18:17:59 2015: Starting NFS common utilities: statd idmapd.
Fri Jan 16 18:17:59 2015: Cleaning up temporary files....
Fri Jan 16 18:17:59 2015: Setting console screen modes and fonts.
Fri Jan 16 18:17:59 2015: ^[]R^[[9;30]^[[14;30]Setting up X socket directories... /tmp/.X11-unix /tmp/.ICE-unix.
Fri Jan 16 18:17:59 2015: Starting iSCSI initiator service: iscsid.
Fri Jan 16 18:18:00 2015: Setting up iSCSI targets:
Fri Jan 16 18:18:00 2015: iscsiadm: No records found
Fri Jan 16 18:18:00 2015: .
Fri Jan 16 18:18:00 2015: Mounting network filesystems:.
Fri Jan 16 18:18:00 2015: INIT: Entering runlevel: 2
Fri Jan 16 18:18:00 2015: Using makefile-style concurrent boot in runlevel 2.
Fri Jan 16 18:18:00 2015: Starting rpcbind daemon...Already running..
Fri Jan 16 18:18:00 2015: Starting NFS common utilities: statd idmapd.
Fri Jan 16 18:18:00 2015: Checking vzevent kernel module .....done
Fri Jan 16 18:18:00 2015: Starting vzeventd:
Fri Jan 16 18:18:00 2015: Starting KSM control daemon: ksmtuned.
Fri Jan 16 18:18:00 2015: Starting enhanced syslogd: rsyslogd.
Fri Jan 16 18:18:00 2015: Starting deferred execution scheduler: atd.
Fri Jan 16 18:18:00 2015: Starting NTP server: ntpd.
Fri Jan 16 18:18:00 2015: Starting RRDtool data caching daemon: rrdcached.
Fri Jan 16 18:18:00 2015: Starting OpenBSD Secure Shell server: sshd.
Fri Jan 16 18:18:00 2015: Starting Postfix Mail Transport Agent: postfix.
Fri Jan 16 18:18:00 2015: Starting pve cluster filesystem : pve-cluster.
Fri Jan 16 18:18:00 2015: clvmd: cluster not configured.
Fri Jan 16 18:18:00 2015: Starting periodic command scheduler: cron.
Fri Jan 16 18:18:00 2015: Starting PVE firewall logger: pvefw-logger.
Fri Jan 16 18:18:00 2015: Starting OpenVZ: ..done
Fri Jan 16 18:18:00 2015: Bringing up interface venet0: ..done
Fri Jan 16 18:18:00 2015: Starting Proxmox VE firewall: pve-firewall.
Fri Jan 16 18:18:00 2015: Starting PVE Daemon: pvedaemon.
Fri Jan 16 18:18:01 2015: Starting PVE Status Daemon: pvestatd.
Fri Jan 16 18:18:01 2015: Starting PVE API Proxy Server: pveproxy.
Fri Jan 16 18:18:01 2015: Starting PVE SPICE Proxy Server: spiceproxy.
Fri Jan 16 18:18:03 2015: Checking if zfs userspace tools present.
Fri Jan 16 18:18:03 2015: Loading kernel ZFS infrastructure: .
Fri Jan 16 18:18:03 2015: Importing ZFS pools.
Fri Jan 16 18:18:03 2015: Mounting ZFS filesystems not yet mountedcannot mount '/tank': directory is not empty
Fri Jan 16 18:18:03 2015: cannot mount '/tank/IMAGES': directory is not empty
Fri Jan 16 18:18:03 2015: failed!
Fri Jan 16 18:18:04 2015: Checking if zfs userspace tools present.
Fri Jan 16 18:18:04 2015: Sharing ZFS filesystems.
Fri Jan 16 18:18:04 2015: Starting VMs and Containers
Fri Jan 16 18:18:04 2015: Starting VM 105