Since Windows 2008 and certain versions of 2003, automounting of iSCSI volumes is enabled by default. While this is fine for most setups, it is sometimes required that the volumes on your iSCSI targets are not automatically mounted when the server starts up (e.g. on a VMware backup server where you don't want to mount VMFS volumes).
There are several ways to disable automount.
Open an elevated command prompt and run either:
diskpart automount disable
This will turn off automount so volumes will not be mounted automatically on a next reboot.
You can verify (and set) the automount status through the registry. Look at the value of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MountMgr\NoAutoMount: if it's set to 1, automount is disabled, else, it's enabled.