Don't wake up external hard disk when saving to internal one
possibilities.

A: Unmount the disk from the command line.  

diskutil unmountDisk Lacie


would be the command to unmount a disk that has a file system named Lacie on it. 
You can also refer to the device itself, but devices can vary from one mount to another.  unmountDisk will take any file system name as the object but will unmount all other file systems if the drive has more than one.

Then, when you want to do a backup, 

 diskutil mount Lacie


You also need to disable spotlight from indexing this drive.