diff --git a/src/PamguardMVC/PamDataBlock.java b/src/PamguardMVC/PamDataBlock.java
index 97236384..28ce0e07 100644
--- a/src/PamguardMVC/PamDataBlock.java
+++ b/src/PamguardMVC/PamDataBlock.java
@@ -987,6 +987,23 @@ public class PamDataBlock<Tunit extends PamDataUnit> extends PamObservable {
 			}
 		}
 	}
+	
+	public void clearAll(boolean andDownStream) {
+		clearAll();
+		if (andDownStream) {
+			int nObs = countObservers();
+			for (int i = 0; i < nObs; i++) {
+				PamObserver obs = getPamObserver(i);
+				if (obs instanceof PamProcess) {
+					PamProcess proc = (PamProcess) obs;
+					ArrayList<PamDataBlock> procOuts = proc.getOutputDataBlocks();
+					for (PamDataBlock b : procOuts) {
+						b.clearAll(andDownStream);
+					}
+				}
+			}
+		}
+	}
 
 	/**
 	 * Reset a datablock. This is called at PamStart from PamController It's been
diff --git a/src/offlineProcessing/OfflineTaskGroup.java b/src/offlineProcessing/OfflineTaskGroup.java
index 7ad4fe41..fd1a4401 100644
--- a/src/offlineProcessing/OfflineTaskGroup.java
+++ b/src/offlineProcessing/OfflineTaskGroup.java
@@ -676,9 +676,11 @@ public class OfflineTaskGroup implements PamSettings {
 			 */
 
 			/**
-			 * Now process the data
+			 * Now process the data. Copy to preserve data integrity. 
 			 */
-			it = primaryDataBlock.getListIterator(0);
+			ArrayList dataCopy = primaryDataBlock.getDataCopy();
+			it = dataCopy.listIterator();
+			
 			unitChanged = false;
 			int totalUnits = 0;
 			int unitsChanged = 0;