@lewisj,
By default , minimum disk space required for splunk to run is 5000 MB(approx 5 GB). In your case, it looks like the partition '/opt/splunk/var' does not have enough disk space and hence halted. To get around this, you may add more disk to this partition or delete any unwanted files or move splunk to other partition where it has enough disk space. Other option (not so recommended) is to reduce the default disk space required from 5GB to lesser by changing below setting in server.conf
[diskUsage
minFreeSpace = <num>|<percentage>
* Minimum free space for a partition.
Reference : http://docs.splunk.com/Documentation/Splunk/latest/Admin/Serverconf#Disk_usage_settings_.28for_the_indexer.2C_not_for_Splunk_log_files.29
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