When looking to upgrade a MacBook Pro with a solid state drive (SSD), one option is to:

At present, many websites (if not most) suggest to do it this way. However, I am concerned about the safety to the spinning HDD (hard disk drive) when in the SuperDrive slot.

The main bay has a sudden-motion-sensor (SMS). It is an accelerometer that detects when the MacBook is falling and prevents damage to the HDD as it is spinning by retracting the reader arm that could scratch the hard disk — damaging/corrupting data.

Although the SuperDrive bay may have the same SATA connector as the main bay (depends on the year of your machine, some have a combination of SATA II and III), it is suggested that it may not have an SMS, which can be a pretty serious issue:

Why not install the SSD in the regular hard drive bay? Good idea! The only problem is that the regular hard drive bay is the only bay that features sudden motion protection. If you drop your MacBook, it’s smart enough to safely park your non-SSD hard drive so it won’t be damaged by the impact. The optical bay interface has no such feature. This is harmless to your ... SSD because it has no moving parts, but any spinning platter-based drive [HDD] will be at risk if installed in the optical bay.

You can have it both ways if your secondary hard drive has its own native sudden motion detection (the ... factory-installed MacBook hard drive [does not]).

But posts on a forum suggest otherwise.

Which do we believe?

If the optical bay lacks the shock sensor, then it would likely be better to leave the HDD where it is, and install the SSD in the optical bay, but this method seems to have an issue with waking from hibernation and sleep.

  • Not sure if this would work, but what if you got a hard drive with a built in sensor and put it in the optical bay?
    – Kevin Chen
    Jan 7, 2013 at 7:43
  • @KevinChen, I already have both a decent HDD and SSD, not looking to buy more at the moment.
    – Baumr
    Jan 7, 2013 at 10:13

2 Answers 2


Some drives have the sudden motion sensor (SMS) built-in. On my 2011 15" Macbook Pro the Toshiba drive has it built-in. I can confirm this because when I simulate a 'drop' I can hear the drive heads park. When I disable SMS through Trim Enabler and do the drop, there is no clicking.

My Toshiba sits in the opti-bay. My SSD sits in the drive bay.

I am reporting severe battery drain (I still keep my user data on my Toshiba drive) and it seems my macbook is heating up more too -- Diablo 3 and SC2 FPS is dropping when my fans are going full blast. I don't recall that happening before the opti-bay upgrade.

  • Did the Toshiba drive come with your MacBook Pro?
    – Baumr
    Sep 10, 2014 at 12:06

This solution may not directly answer your question, but it may be relevant: I would advise you against putting your HDD in the optical bay.

I tried that. Putting the SSD in place of the HDD, and moving the HDD in the optical bay, which is just under the keyboard… And I got to hear the noise of the HDD spinning and the noise the heads when accessing cylinders… Bad idea. After a few hours, I switched HDD and SSD.

So, I would recommend you to leave the HDD in its bay and to put the SSD in the optical bay. That way your question is solved, and you don't get to hear the HDD working. And if you notice an extra delay when booting, just select the SSD (which is in the optical bay) as booting drive in the system preferences. You will need to enter your password twice, once and for all. Then, your mac will immediately find the SSD when starting up.

  • 1
    I may be wrong (english is not my native language) but I thought your question was more about the sudden motion sensor – is it OK to put the HDD in the optical bay, or is it lacking the SMS ? Strictly speaking, the answer I provided does not match the question. But I thought it could be useful, since, IMHO, it shows that putting the HDD in the optical bay is not an option, regardless of the presence of SMS there. Maybe I should rephrase: This solution may not directly answer your question, but…
    – Jean
    Jan 29, 2013 at 0:55
  • It's done. Answer edited.
    – Jean
    Jan 29, 2013 at 10:03

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