MacOS – Upgrade to Yosemite with two hard drives in iMac

So, by virtue of:

this question is not about third party TRIM Enabler and the like

This cannot be the "right answer" it would seem. But even if thats not what the question is about, that is what the, um, "correct answer" is kind of about? So I'll give it a shot anyway...

The short answer to the title question: in short, no.

There is only one implementation... per supported controller.

hfs_unmap_free_ext doesn't refer to the hfs_unmap_free extent function.

It refers to the kext which does the "hfs_unmap_freeing."

The error,

hfs_unmap_free_ext: ignoring trim can occur for one or both of two reasons:

• The drive is one that uses a supported controller, but has not been "blessed" with an Apple serial number and firmware. In this case, patching this protection out the kernel extension - what Trim Enabler does - will enable TRIM since the hardware has a driver which supports it.

  (Also, myself, I would consider patching the kernel to be almost the definition of something intergral to the operating system.)

• It is ignored because although the device advertises that it supports TRIM, there's no TRIM driver in the kernel for that controller, since Apple never shipped a drive based on that controller in the first place. So it just uses generic ATA commands.

(This is probably the case with your drive, due to the "not recognized as an SSD" bit.)

While:

they have programmed in nanosecond-critical timing loops that match perfectly with the access timings of the controllers used in Apple’s SSDs

is a true statement, that's definitely not why Apple locks drives. They do that so you can't get in-warranty support for a third party drive.

Apple doesn't make drives; but if you read up how Tim Cook (while COO) cornered the Asian NAND flash market a few years back - more or less why he's CEO today - you'll understand why flash manufacturers allow Apple to dictate stuff like this.

Other than that, I'm not sure you're going to find much more than that thats "authoritative" I'm afraid. Like you said, most of this stuff is closed source, though you can find traces littered everywhere, with includes like this one. Also, note that TRIM is more of a concept/implementation thing than a standard (at least with anything prior to SATA 3.1).

I can also confidently tell you that TRIM enabled/Apple-logo branded SSDs can only be purchased through GSX (or the grey market). And that Trim Enabler works quite well. I've installed (the more recent versions) in many a production environment.

A couple of links:
http://www.anandtech.com/show/5453/trim-enabler-20-for-os-x-lion-released http://en.wikipedia.org/wiki/Trim_(computing)

re 1: hard to say - if you can spare the Money you should go for the 850 bc. of a (probably) better Garbage Collection (see #3), else go for the 840.

re 2: The best tutorial for replacing the optical drive with a harddrive (or SSD) can be found at iFixit.com, f.e. this one. You should search for the Tutorial for your model, the link i posted is probably not for your model.

re 3: There is a hot discussion going on if Trim should be enabled or not and if that is a bad thing. Fact is, Apple has Trim Support enabled only on its own OEM SSDs and disabled it for third party drive. There are Tools to enable TRIM Support, but it could be a not necessary bc. of a better Garbage Collection within the SSD Firmware (see this article and this Article). For general Infos i recommend this Article from Samsung

BTW, to create a homemade Fusion Drive you need to use the 'Core Storage Volume' Command. You can look it up with

man diskutil

look at the 'coreStorage' section. There are some tutorials for creating a homemade Fusion Drive, but it's a very painful process.