"To start with it turns out the system is very similar to the older 2015 model so a full teardown won't offer much from a repair perspective.
Yes, understanding the internals can be useful for future upgrading! So to help you see the insides, take a look at this video: Unboxing and Teardown of the 2017 27"" iMac retina 5k OWC did.
You may want to also review the EveryMac listings: 27"" iMac's 5K (Mid-2017)
As for upgrading the HD with either your own HDD or SSD drive you will need the OWC in-line sensor: OWC In-line Digital Thermal Sensor for HD Upgrade (HDD or SSD) for 27"" iMacs 2012 and Later
The CPU does appear to be upgradable (socketed)
Not sure I understand what you mean by 'Phases' are you asking how many cores or threads the CPU offers? The EveryMac listings should answer that based on what model you are thinking of as well as the GPU and VRAM each uses.
To understand the CPU's details I would recommend you visit Intel's ARK site. Here is a breakdown of the major three:
"NVMe M.2 drives do work with the iMac 5K 2015 and 2017 as well as the Macbook Air 13"" 2015 and the Macbook Pro Retina 13"" 2015 out of the box with the correct adaptor. It appears that only particular versions and brand of the adaptor works well.
The 2013-2014 Macs need a modded firmware to work well.
@danj has mentioned in another thread that adaptors might have massive CRC issues, but he did the test 1 year back. No sure if the adaptors that the Macrumors users are using are newer versions that seem to work very well.
Check out this link. Someone used a Samsung EVO 970 drive in his iMac 5K 2017 without any issues.
The HDD is upgradable without a thermal sensor if you use a temperature control app.
You can use Macs Fan control or SSD fan control. Both will work.
The SSD only version comes with a 3.5"" Hard Drive bracket but not the SATA cable."
"I have successfully upgraded my iMac (Retina 5K, 27-inch, 2017) with an Intel i7-7700 and a 2TB Samsung 970 EVO NVME SSD. Please see post below for full details.
This doesn't seem to be the place to tell people to just buy the better one... We are here for a reason. We already have it and want to upgrade it. Maybe at the time it was purchased upgrading hadn't gotten to where it is now.
