diff --git a/reactos/drivers/filesystems/fastfat/fat.c b/reactos/drivers/filesystems/fastfat/fat.c
index c9fb49ea57d..8ff955a4fbd 100644
--- a/reactos/drivers/filesystems/fastfat/fat.c
+++ b/reactos/drivers/filesystems/fastfat/fat.c
@@ -164,6 +164,8 @@ FAT16FindAndMarkAvailableCluster(PDEVICE_EXTENSION DeviceExt,
           *Block = 0xffff;
           CcSetDirtyPinnedData(Context, NULL);
           CcUnpinData(Context);
+          if (DeviceExt->AvailableClustersValid)
+            InterlockedDecrement((PLONG)&DeviceExt->AvailableClusters);
           return(STATUS_SUCCESS);
         }
 
@@ -227,6 +229,8 @@ FAT12FindAndMarkAvailableCluster(PDEVICE_EXTENSION DeviceExt, PULONG Cluster)
 	       *CBlock = (*CBlock & 0xf) | 0xfff0;
 	     CcSetDirtyPinnedData(Context, NULL);
 	     CcUnpinData(Context);
+	     if (DeviceExt->AvailableClustersValid)
+	       InterlockedDecrement((PLONG)&DeviceExt->AvailableClusters);
 	     return(STATUS_SUCCESS);
 	   }
     }
@@ -281,6 +285,8 @@ FAT32FindAndMarkAvailableCluster (PDEVICE_EXTENSION DeviceExt, PULONG Cluster)
           *Block = 0x0fffffff;
           CcSetDirtyPinnedData(Context, NULL);
           CcUnpinData(Context);
+          if (DeviceExt->AvailableClustersValid)
+            InterlockedDecrement((PLONG)&DeviceExt->AvailableClusters);
           return(STATUS_SUCCESS);
         }