Unable to restore Partition from USB Drive

After a crash drive,Comodo failed to restore my entire C partition.
Here are the steps I have done:

  1. Boot on USB key with Windows 7 installation on it.
  2. After re-installing Windows,I ran Comodo to restore from my system Backup.
  3. It said the disk was “in use” and planned a restore at next re-boot.
  4. After re-booting Comodo said he cannot restore from my USB drive !!

So I can boot and install Windows from my USB key,but Comodo cannot use my USB drive for restoring ??
So my system back up on an external drive is useless !