How to turn off spindle of internal HDD on MacBook from command line?
to disable the drive from mounting during boot :
Find the uuid of the drive (in terminal):
diskutil info /Volumes/[name]
Then create a new file called /etc/fstab and in it put
UUID=34DFD44D-489C-1C22-01D8-DFDF288C102F none hfs rw,noauto 0 0
just replace the UUID above with the actual UUID of the drive.
Use the following command to view a list of drives/partitions
diskutil list
And then issue the mount command to mount the drive
sudo mount /dev/disk1s1 /Volumes/DriveName
Obviously change the disk id and mount folder to suit your system.