I have a 27” iMac late 2012 that I’m swapping out the OEM SATA III 3.5

@danj Thanks for the clarifications and additional details. Too bad that the inline thermals sensor sata cable costs appx 2x what i spent on the new ssd. What if i were to leave the existing apple oem 3.5" hdd in place and use a low cost sata cable splitter [ e.g. https://smile.amazon.com/dp/B07FCCTFNN/ for $7.69 ] to enable internal connection to new ssd and just setup system to boot from the new ssd only. Would the existing apple 3.5" hdd connection manage fan control in that case with the new 2.5" ssd zip tied to it?

@myusrn olk - Don't see it working here ;-{ SATA is not PATA no chaining possible. You do know you do have a second port the custom mSATA blade drive connector iMac Intel 27インチ EMC 2546 SSD 交換 Why don't you just get that and set it up as your boot drive.

@danj Thanks for the addition suggestions. Are we saying that with iMac 27" late 2012 it is perhaps better to go with a low cost pciE m.2 2280 sata iii ssd and use that instead of a low cost 2.5" sata iii [ or nvme ??? ] ssd storage device?

If i go that route and remove the apple oem 3.5" hdd i presume the system isn't going to be doing any related fan activity with it disconnected altogether.

It seems adapters such as [ https://ebay.com/sch/i.html?_from=R40&_t... ] are required to plug a standard issue m.2 2280 ssd to the fusion drive "blade ssd" connector but no insights in those listings if you use pciE m.2 2280 nvme or m.2 sata iii with the adapter.

When i look at https://everymac.com/systems/apple/imac/... it has the following <snip> suggesting that even if my iMac 27" late 2012 didn't originally ship with a fusion drive setup it will still have a pciE m.2 2280 sata iii ssd connector.

<snip>By contrast, the 27-Inch "Late 2012" and "Late 2013" as well as the "Late 2014" and "Mid-2015" Retina/5K iMac models all have this connector on the motherboard even if the system only is configured with a hard drive at the time of purchase, so it is possible to add a PCIe-based "blade" SSD to these models after the initial system purchase</snip>

@myusrn olk - Don't get distracted with newer systems as they don't apply here.

The 2012/early 2013 models are both mSATA blade drive models, the late 2013 onwards are NVMe/PCIe x2 or x4 blade drive. Review this for the deeper details The Ultimate Guide to Apple’s Proprietary SSDs. And the 27" models ALL have the blade drive interface, ONLY some 21.5" models have it missing.

I would go with either the Apple or OWC blade SSD's. Last year I cleaned out a drawer full of adapter and M.2 SSD's all coming from iMac's and MacBook Pro's. They don't hold up as Apple has special lines to the SSD which these adapters and M.2 have no clues! These involve how Apple handles Garbage collection, TRIM & Wear-leveling which go beyond how M.2 standards work.

@danj Great information and also in article link you shared, thanks.