IMac – External SSD with OS installed not working

Definitely make the SSD your boot/app disk and set your computer to put hard disks to sleep. But even then there are many factors that can cause the drive to spin up even. Sometimes just getting an open dial box will cause it to spin up.

I'd first make sure your HDD is not shown in the sidebar - if it is there it will definitely spin up every time you get a save or open dialog box or a finder window.

Even after doing that I still had it spinning up more than I wanted so I followed this blog post on Dae’s blog to make an apple script that will unmount or mount it so I can explicitly control when it sleeps or doesn't sleep. Incase the link dies, here is the script. It even does Growl notification:

set myVolumeLabel to "Storage HD"

tell application "GrowlHelperApp"
    set the allNotificationsList to {"Disk mounted", "Disk unmounted"}

    register as application "Disk Mounter" all notifications allNotificationsList default notifications allNotificationsList icon of application "Disk Utility.app"
end tell

tell application "Finder"
    set diskDev to do shell script "diskutil list | grep \"" & myVolumeLabel & "\" | grep -o 'disk[0-9]*' "
    if not (disk myVolumeLabel exists) then
        do shell script "diskutil mountDisk " & diskDev
        tell application "GrowlHelperApp" to notify with name "Disk mounted" title "Volume mounted" description "Disk “" & diskDev & "” with volume “" & myVolumeLabel & "” has been successfully mounted." application name "Disk Mounter" icon of application "Disk Utility.app"
    else
        do shell script "diskutil unmountDisk " & diskDev
        tell application "GrowlHelperApp" to notify with name "Disk unmounted" title "Disk unmounted" description "Disk “" & diskDev & "” with volume “" & myVolumeLabel & "” has been successfully unmounted." application name "Disk Mounter" icon of application "Disk Utility.app"
    end if
end tell

One thing to note - this unmounts ("ejects") an entire drive - not just a single partition.

In your case probably a upgrade to SATA 3 SSD (internal) would do the most.

AFIK your model supports it, and you will get up to 6 Gb/s R/W speeds and combined with RAM upgrade would get you what you want.

Adding a external drive to expand the storage if needed, the one would use the Thunderbolt SSD.

Find out if your Model supports the SATA 3 SSD before spending the money:

Open About this mac- more info- system report - SATA..

then you will see something like I have

My little MacBook Air rocks, it can run up to 6 Gb/s