diff --git a/gcimagebundle/gcimagebundlelib/block_disk.py b/gcimagebundle/gcimagebundlelib/block_disk.py
index <HASH>..<HASH> 100644
--- a/gcimagebundle/gcimagebundlelib/block_disk.py
+++ b/gcimagebundle/gcimagebundlelib/block_disk.py
@@ -90,7 +90,10 @@ class FsRawDisk(fs_copy.FsCopy):
     # Verify there is only 1 partition on the disk
     with utils.LoadDiskImage(file_path) as devices:
       # For now we only support disks with a single partition.
-      if len(devices) != 1:
+      if len(devices) == 0:
+        raise RawDiskError(
+            'Device %s should be a disk not a partition!' % self._disk)
+      elif len(devices) != 1:
         raise RawDiskError(
             'Device %s has more than 1 partition. Only devices '
             'with a single partition are supported.' % self._disk)
Update block_disk.py

in case you specify gcimagebundle -d /dev/sda1

the message is not leading you to specify a disk, it makes you confused about why a single partition

is not a single partition