It really shouldn’t get stuck. In my experience this usually only happens if there is corruption in the storage_usage database. If this happens and it sees less than 500 GB free space, the temporary work around to get the node to start (which will make it recalculate space used) is to stop the node, set the storage2.monitor.minimum-disk-space: 500.00 GB parameter in the config.yaml to a lower value (and make sure to uncomment the line), and restart.
(and as a side note, make sure not to confuse the “storj database” and used storage space, as these mean different things. I am assuming your iSCSI slice both has the databases and storage.)