When using iSCSI, SMB, NFS, ... it is possible that it is trying to mount it before the network is activated. Obviously this will not work.
Luckily there is a fix for this, in the /etc/fstab file you can add _netdev as an option to mount it after the networking has started.
For example:

<NETWORK_DRIVE> /mnt/drive2 ext4 defaults,_netdev 0 0