f you have multiple hard drives connected to your Mac, or maybe you have multiple partitions on a single hard drive, by default, OS-X will mount all those volumes at boot time.
In case you don’t want this to happen, and instead mount one only on-demand, when needed, maybe with Disk Utility, here is what you need to do.
Start by opening the Disk Utility app in your /Applications/Utilities folder. Select the drive or partition you don’t want to have auto mounted and click the info button. Now copy the Universal Unique Identifier string and open the terminal, edit or create the
/etc/fstab file and insert the following statement, replacing the UUID with yours of course:
sudo nano /etc/fstab UUID=12345678-1ABC-1CDE-1FGH-123456789012 none hfs rw,noauto