Installing SSD is a heat sensor required?

Welcome!

First, what drive configuration does the computer now have? We need a starting point to avoid jumping to conclusions.

it looks easy enough...

Interesting. I know some trained techs who won't open a 2012 or later iMac due to risk of damage, or feeling that the customer will balk at a two-hour or higher labor charge. That is a sealed-case design; it is the cutting of the seal that can also cut something far more important.

The OWC listing of an SSD upgrade kit for your model does not show the heat sensor cable.

https://eshop.macsales.com/item/OWC/K21IM12HE500/

I believe the last models that required it were the 2011 iMacs.

Per OWC, you cannot install a blade-type SSD unless the computer came with a factory SSD or a factory Fusion drive. Otherwise the slot is either not there or not connected. That is why we need the current drive config.

You can, however, use a 2/5-inch SSD in the existing drive bay. If you make sure the SSD is rated SATA 6GB/sec, the SSD transfer speeds will be about 500MB/sec, some 8-10X faster than what most Apple factory roto-drives can do.

The sane and safe approach that does not involve opening something Apple didn't what opened is to get a USB3 external drive enclosure and put the 6GB/sec SSC in that. Connect the external to your Mac and use Carbon Copy Cloner to CLONE, not copy, you entire internal drive to the external. Then use Startup Disk prefs to set the external as your boot volume.

That will give your three very positive things:

1) transfer speeds with be around 400 MB/sec and that won't feel slow at all.

2) You will already own a compatible SSD if you decide to rip open the computer and do the internal install.

3) You can continue to use the external in the future if you get a newer computer, Mac or Windows.

NOTE: Most senior contributors here recommend either Crucial (MX series, please) or OWC 2.5-inch SSDs for Macs. Many of the super-cheap ones will not run at rated speeds under OSX, and I am seeing a concerning number of speed problems posted here involving newer Samsung SSDs like the EVO 860.

I completely agree with @Allan Jones.

DaveInUK wrote:

it looks easy enough

I can assure you that videos do make it appear quite easy. I can tell you from personal experience that it can be quite difficult. The cables and connectors are very delicate and easily damaged. There is very little room to see & work to disconnect and reconnect these very fragile cables/connectors. I become frustrated working on these iMacs for this reason. Plus the LCD Panel can be cracked if you are not careful in how you store it while working on the iMac. It is best to keep the LCD Panel vertical and carefully lean it against something so it won't slide or fall down (I like to place it on end between two objects so there is no chance of the panel slipping). If you place the LCD Panel "flat" on a table you may end up having the panel crack under its own weight since the back of the panel has cables attached so the panel will not be flat on the table (happened with one our own techs). Placing the panel face down risks scratching the glass surface.

Once the panel is removed it is a bit easier to work on the iMac, but it can still be tricky depending on the model. You must be extremely careful not to touch the exposed power supply as it can retain very dangerous voltage even many hours after the iMac has been unplugged.

Good luck if you decide to attempt the procedure.

Good morning Dave. Thanks for the followup.

1) I don't know the actual drive configuration at the moment because I won't get the computer for a couple of weeks.

Perhpas we should wait to see what drive option is fitted to this computer. If:

a) it has Apple's Fusion Drive or an SSD now, and

b) you promise that you understand our cautions about opening an iMac,

then the blade SSD becomes a much faster internal option than a 2.5-inch SSD. It could be up to 5X faster as the 2.5-inch types.

Remember that Apple's incarnation of a hybrid drive consists of a small SSD and a conventional roto-drive linked by software. Thus an iMac with the factory Fusion option has an available slot for a larger blade SSD.

2) That definitely puts me in the insane group.

Excellent! So that's why I like you! Curiosity is good—normal is boring. The adhesive is a strip of material that has to be replaced. Fortunately, the reseal kits are available. See the link to the "kit" in my first response.

4) I take on-board the advice of SSD drives and am slightly concerned about your comments on the Samsung because I've already bought the Samsung V-NAND 860 PRO 2.5"SSD.

I think the collective experience here is that we see far more issues reported with Samsung than Crucial or OWC SSDs. Unless we are professional repair people like our very sage HWTech, the weight of trouble reports here is all we have to go on. Perhaps HWTech can share experiences in actually servicing different SSD types. I work on my family's tech and pro bono on those of a few Mac-owning friends, but am by no account a "pro."

Crucial have a UK site, and OWC have UK partners and distributors:

https://www.owcdigital.com/reseller/resellers#resellers

The OWC main page has excellent instructional videos.

If you have those types of technical skills, then you will probably do Ok. The iMac is just more awkward than anything else since you have to juggle the heavy display while disconnecting & connecting those hard to reach very delicate cables/connectors. Most of my iMac experience has been with 2011 and earlier iMacs although I have worked on a few later model iMacs. Just be very careful when cutting into the adhesive to separate the Display from the frame since I think there is one area where you could accidentally slice something important IIRC. I used the pizza cutter style wheel made specifically to remove the iMac Display so using anything else may be a bit tricky since you don't want to crack the glass or cut into a cable or the LCD.

Most of my experience with third party SSDs has been with Crucial MX series or OWC Mercury 2.5" SATA SSDs. I definitely prefer the Crucial MX series SSDs over the OWC Mercury SSDs for a number of mostly personal reasons. I don't have a lot of experience with Samsung SSDs. I am planning on building a personal PC for the first time in many many years and I am seriously looking at the Samsung M.2 SSDs (some of my decision will be based on whether Samsung continues to provide easily accessible OS independent SSD firmware updaters since I'm not sure the Crucial M.2 SSDs are as good -- still need to research the M.2 SSDs some more since for work we usually just need a good compromise on price & performance where Crucial definitely wins). Most of the issues with the Samsung SSDs in Macs tend to be related to the older EVO series in older Macs I believe (as seen in reports on the Apple forums). Keep in mind most of the Apple branded SSDs I have seen are usually Samsung based (higher end ones of course). These are definitely three of the better more reliable brands and models out there which include OS independent SSD firmware updaters (very important for me as a Linux user and when dealing with Macs).

One thing you can count on with SSDs of any brand is that they can fail at any time without any warning signs so make sure to always have frequent and regular backups.

Good luck with your upgrade.

Thanks for the follow up. I'm glad you were successful in your upgrade.