Don't wake up external hard disk when saving to internal one My problem is that I have an external hard drive connected to my Mac. Since it is only used for backup purposes, it is sleeping most of the time. 

However if I now want to save something (e.g. a picture from a website) to my internal hard disk, I have to wait for the external one to wake up before the "save as..." dialogue appears. 

I know about the question How to save a file quickly and no need to wait for the external hard drives to &quot;wake up&quot;?, but it sounds like it intends a solution about not letting the hard disks go to sleep whereas I would like to let the external hard drive have its well-deserved rest.  



EDIT:


I do not want to unplug it, because I know that I will forget to reconnect it before the automatic backup
various other operating systems deal very well with situations like that




EDIT2:

Using Sherwood Botsford's answer I put together the following AppleScript which mounts the hard drive, executes for example an rsync and unmounts it again.

do shell script "diskutil mount 'Name of your Volumn'"

do shell script "rsync -av --delete ~/ '/Volumes/Name of your Volume/.'"

do shell script "diskutil unmountDisk 'Name of your Volume'"
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.