diff --git a/run/disk.sh b/run/disk.sh
index a8960a0..86b9c08 100644
--- a/run/disk.sh
+++ b/run/disk.sh
@@ -29,7 +29,7 @@ if [ -f "${DATA}" ]; then
 
     echo "INFO: Resizing data disk from $OLD_SIZE to $DATA_SIZE bytes.."
 
-    if [ "$ALLOCATE" != "Y" ]; then
+    if [ "$ALLOCATE" = "N" ]; then
 
       truncate -s "${DATA_SIZE}" "${DATA}"; 
 
@@ -45,10 +45,20 @@ if [ -f "${DATA}" ]; then
         echo "ERROR: Specify a smaller size or disable preallocation with ALLOCATION=N." && exit 84
       fi
 
-      if ! fallocate -l "${DATA_SIZE}" "${DATA}"; then
-        echo "ERROR: Could not allocate a file for the virtual disk." && exit 85
-      fi
+      if [ "$ALLOCATE" = "Z" ]; then
 
+        GB=$(( (REQ + 1073741823)/1073741824 ))
+        echo "INFO: Writing ${GB} GB of zeroes, please wait.."
+
+        dd if=/dev/zero of="${DATA}" seek="${OLD_SIZE}" count="${REQ}" bs=1M iflag=count_bytes oflag=seek_bytes
+
+      else
+
+        if ! fallocate -l "${DATA_SIZE}" "${DATA}"; then
+          echo "ERROR: Could not allocate a file for the virtual disk." && exit 85
+        fi
+
+      fi
     fi
   fi
 
@@ -66,7 +76,7 @@ if [ ! -f "${DATA}" ]; then
 
   # Create an empty file
 
-  if [ "$ALLOCATE" != "Y" ]; then
+  if [ "$ALLOCATE" = "N" ]; then
 
     truncate -s "${DATA_SIZE}" "${DATA}"
 
@@ -80,16 +90,25 @@ if [ ! -f "${DATA}" ]; then
       echo "ERROR: Specify a smaller size or disable preallocation with ALLOCATION=N." && exit 86
     fi
 
-    if ! fallocate -l "${DATA_SIZE}" "${DATA}"; then
-      rm -f "${DATA}"
-      echo "ERROR: Could not allocate a file for the virtual disk." && exit 87
-    fi
+    if [ "$ALLOCATE" = "Z" ]; then
 
+      echo "INFO: Writing ${DISK_SIZE} of zeroes, please wait.."
+
+      dd if=/dev/zero of="${DATA}" count="${DATA_SIZE}" bs=1M iflag=count_bytes
+
+    else
+
+      if ! fallocate -l "${DATA_SIZE}" "${DATA}"; then
+        rm -f "${DATA}"
+        echo "ERROR: Could not allocate a file for the virtual disk." && exit 87
+      fi
+
+    fi
   fi
 
   # Check if file exists
   if [ ! -f "${DATA}" ]; then
-    echo "ERROR: Virtual DSM data disk does not exist ($DATA)" && exit 88
+    echo "ERROR: Virtual disk does not exist ($DATA)" && exit 88
   fi
 
   # Format as BTRFS filesystem
@@ -97,6 +116,13 @@ if [ ! -f "${DATA}" ]; then
 
 fi
 
+# Check the filesize
+SIZE=$(stat -c%s "${DATA}")
+
+if [[ SIZE -ne DATA_SIZE ]]; then
+  echo "ERROR: Virtual disk has the wrong size: ${SIZE}" && exit 89
+fi
+
 AGENT="${STORAGE}/${BASE}.agent"
 [ -f "$AGENT" ] && AGENT_VERSION=$(cat "${AGENT}") || AGENT_VERSION=1
 
diff --git a/run/install.sh b/run/install.sh
index b50a911..2e98307 100644
--- a/run/install.sh
+++ b/run/install.sh
@@ -106,7 +106,7 @@ unzip -q -o "$BOOT".zip -d $TMP
 echo "Install: Creating partition table..."
 
 SYSTEM="$TMP/sys.img"
-SYSTEM_SIZE="4954537983"
+SYSTEM_SIZE=4954537983
 
 # Check free diskspace
 SPACE=$(df --output=avail -B 1 "$TMP" | tail -n 1)
@@ -115,9 +115,35 @@ if (( SYSTEM_SIZE > SPACE )); then
   echo "ERROR: Not enough free space to create a 4 GB system disk." && exit 87
 fi
 
-if ! fallocate -l "${SYSTEM_SIZE}" "${SYSTEM}"; then
+if [ "$ALLOCATE" != "Z" ]; then
+
+  # Cannot use truncate here because of swap partition
+
+  if ! fallocate -l "${SYSTEM_SIZE}" "${SYSTEM}"; then
+    rm -f "${SYSTEM}"
+    echo "ERROR: Could not allocate a file for the system disk." && exit 88
+  fi
+
+else
+
+  GB=$(( (SYSTEM_SIZE + 1073741823)/1073741824 ))
+  echo "INFO: Writing ${GB} GB of zeroes, please wait.."
+
+  dd if=/dev/zero of="${SYSTEM}" count="${SYSTEM_SIZE}" bs=1M iflag=count_bytes
+
+fi
+
+# Check if file exists
+if [ ! -f "${SYSTEM}" ]; then
+    echo "ERROR: System disk does not exist ($SYSTEM)" && exit 89
+fi
+
+# Check the filesize
+SIZE=$(stat -c%s "${SYSTEM}")
+
+if [[ SIZE -ne SYSTEM_SIZE ]]; then
   rm -f "${SYSTEM}"
-  echo "ERROR: Could not allocate a file for the system disk." && exit 88
+  echo "ERROR: System disk has the wrong size: ${SIZE}" && exit 90
 fi
 
 PART="$TMP/partition.fdisk"