Is installing a HDD instead of SuperDrive safe?

It's pretty popular to replace ones optical drive* on the MacBook Pro with a HDD, but how serious are the risks?

^{*In most cases, you'd do this when installing a third-party SSD, but I'd like to leave any of those benefits out of this and isolate matters of the HDD and Superdrive.}

I mean things like:

• Overheating
• Performance issues
• Battery life
• Warranty status
• Weight increase
• Upsetting Steve

Update: The above questions have been addressed in this question, but there is a more serious issue — potentially, a missing sudden motion shock sensor in the optical drive.

Solution 1:

• Overheating shouldn't be an issue. The SuperDrive probably runs hotter (when it's in use) since it has moving parts
• An SSD would most likely improve performance (especially if you use it as your boot drive and place for applications)
• Don't know about battery life, but I imagine if you you use this as your boot drive, and the stock hard disk as your "slave", you might actually see an improvement (again, fewer 'moving parts')
• This will void your warranty, but if you're careful you might be able to get in and out without leaving evidence of tampering. Consult iFixit's guides for tips
• I can't imagine there would be a noticeable weight difference.
• Steve is dead, unless you're talking about Steve Zahn. Steve Zahn doesn't care what you do with your computer.

Solution 2:

I've done this to my own MacBook Unibody Late '08 earlier this year. In answering each query, this is what I've noticed...

• Overheating: My machine has recently started over heating but it's hard to attribute this to the SSD/HDD combo. Most heat is coming not from where the SATA bays are located, but rather where the USB ports etc. sit.
• Performance: has massively improved, but without a shadow of doubt, as the months have gone by, it has deteriorated. I still get lightning boot ups but apps are not as quick as they were. Please not that most 3rd party SSDs under Snow Leopard and above have no TRIM support. I've only just enabled mine through back door command line methods, which as with anything command line, may not be 'ideal' for the light-hearted user.
• Battery life: has definitely decreased. In place of my optical bay is a 7200RPM hard drive, which takes more power than a standard 5400RPM. Also, given that you would be using your HDD a lot, unlike your superdrive which you use more occasionally than constantly (I'm assuming), you can expect some decrease.
• Warranty: Will be severed!
• Weight: the SSD is lighter than the original HDD. However, my new HDD and superdrive weight similar, the HDD 'feeling' heavier.

I've just posted a query about my machine as I think the SSD is causing some issues. The response to that may shed some further light on the topic.

Solution 3:

RE: voiding the warranty. When I recently bought my non-retina 13" MBP, I told several of the Apple Store people that I planned to swap out the HDD for the SSD I took from my now-dead (logic board) 15" MBP A1211, and that I realized I would void the warranty. They all told me that Apple considers the two drive bays "user replaceable" and that doing what I planned would NOT void the warranty. Specifically, I could put any SATA device in either bay. Anyone test this info?

Solution 4:

I can confirm that for the MBP Apple does not have a problem with users changing the drives. It does NOT void the warrantee.

My experience: I have replaced the disk drive with a 512G SSD and replaced the optical drive with a 1TB disk drive. I suspect that airflow is impacted as I feel that I run a bit hotter than before but I'm not really sure.

Battery life is better if I'm not churning the fixed disk (ie only using the SSD or cache).