How to make a Mac harddrive partition non bootable
To check whether a disk drive is bootable, open Terminal (in Applications>Utilities) and type (replace /Volumes/SSD with the folder your disk drive is mounted at):

bless --info /Volumes/SSD/


If the output looks similar to this:

finderinfo[0]:      0 =&gt; No Blessed System Folder
finderinfo[1]:      0 =&gt; No Blessed System File
finderinfo[2]:      0 =&gt; Open-folder linked list empty
finderinfo[3]:      0 =&gt; No alternate OS blessed file/folder
finderinfo[4]:      0 =&gt; Unused field unset
finderinfo[5]:      0 =&gt; No OS 9 + X blessed X folder
64-bit VSDB volume id:  0x844F837C9317318A


your computer won't be able to boot from the drive.

For comparison, this is what the output of a bootable disk drive would look like:

bless --info /

finderinfo[0]:     62 =&gt; Blessed System Folder is /System/Library/CoreServices
finderinfo[1]: 327769 =&gt; Blessed System File is /System/Library/CoreServices/boot.efi
finderinfo[2]:      0 =&gt; Open-folder linked list empty
finderinfo[3]:      0 =&gt; No alternate OS blessed file/folder
finderinfo[4]:      0 =&gt; Unused field unset
finderinfo[5]:     62 =&gt; OS X blessed folder is /System/Library/CoreServices
64-bit VSDB volume id:  0xD9A24BFA7A5E7EDC


The command bless includes option --unbless to unset volume bootability (from man bless):


  --unbless directory  Use the HFS+ volume mounted at directory and unset any persistent blessed files/directories in the HFS+ Volume
                                Header.


So if you find out in the output above that your drive is bootable, "unbless" it:

sudo bless --unbless /Volumes/SDD/