Prevent internal drive from mounting automatically

Solution 1:

1) Make sure the disk you want to prevent mounting at boot is mounted.

2) Launch Terminal.

3) Run the following command to print out information about the disk:

 diskutil info /Volumes/<volume that shouldn't be mounted>

4) Locate the line that starts with: Volume UUID:. Select the UUID (Universal Unique Identifier) that follows on the rest of the line. It will be something that looks like FF9DBDC4-F77F-3F72-A6C2-26676F39B7CE. Your value will be different

5) Copy the UUID to the clipboard.

6) Navigate to /etc by typing the following and pressing enter:

 cd /etc

7) Edit (or create) an fstab file by typing the following and pressing enter:

 sudo vifs

8) Enter the following line, substituting the UUID you copied in step 5). (Note: vifs uses the value of the EDITOR environment variable to pick the text editor to use. This article assumes you are using the default value of vim. In vim, starts in command mode. To add a new line, move to the end of the document, and press the o key to append a new line and enter edit mode. Then type the following and press the return key.):

UUID=FF9DBDC4-F77F-3F72-A6C2-26676F39B7CE none hfs rw,noauto

9) Type escape to return to command mode and then type ZZ to save and exit vifs (or some other method to save and exit if you are using something other than vim.

10) Type the following and press enter to reset the auto mounter:

 sudo automount -vc

11) Quit Terminal

Now, the next time you restart, or unplug the drive in the case of an external drive, that volume will not be mounted. All you have to do is launch Disk Utility, select the grey, disabled volume, and click the Mount button.

Source