I don’t understand fusion drives. Can someone please tell me?

That 2013 iMac has a 3.5" HDD that can be replaced with a 2.5" SATA SSD using a tray and cable kit.

If it is Fusion drive model, then it has a non-standard SSD on the backside of the logic board.

see > https://eshop.macsales.com/shop/ssd/owc/imac-21.5-inch/2013-2019

Considering the 2013 iMac's age and limited ability to run anything later than macOS 10.15 Catalina.

It would be best to just run it from an external SSD and save up money for a new Mac.

see > Use an external SSD as your startup disk … - Apple Community

uras146 wrote:

Does the iMac late 2013 27” have a soldered ssd or a plug in ssd?

Exactly which model do you have? 3.2GHz, 3.4GHz or 3.5GHz CPU? While the 3.5GHz model (EMC 2639) had a 128GB blade SSD there is no guarantee that the other models also had it. Even in the case of the blade SSD, it's mounted on the underside of the logic board ... and accessing it requires a complete teardown. There's no reason to touch it.

I suggest a much better path is to install macOS on an external Thunderbolt 2 or USB 3 drive and use that as your boot drive instead of the Fusion drive. Much simpler to do, and it doesn't involve tearing apart your iMac. You can then use the internal Fusion drive for storage, or just abandon using it.

Final thought ... with a 12 year old obsolete iMac on your hands, it's time to start thinking about replacement.

Running from an External SSD in way cheaper and easier than upgrading either of the internal drives.

Beside that, iMac's are very difficult to work on and if something get busted, then your really SOL...

uras146 wrote:

but I’m broke and Mac’s are too expensive =[

You're going to have to spend at least $100 for any decent 1TB SSD whether it's internal or external. And if you go external you don't risk damaging your iMac by opening it up. Doing the surgery to replace the internal HDD is a delicate operation and you can quickly hose your Mac at any point in the process.

With an external SSD like this one from OWC ($99.99) you can very easily upgrade your setup. If you are concerned about performance with a USB-attached bootable drive you will probably not notice any difference vs. the Fusion drive ... you may even notice some improvement because the APFS disk format is designed for SSDs.

A Fusion Drive combines a small SSD and a large HDD. The idea was to strike a compromise that offered some of the advantages of a SSD (speed) without completely sacrificing the advantages of a HDD (capacity, cost per byte).

Physically,

• The HDD is a standard 2.5" SATA notebook drive or 3.5" SATA desktop drive.
• The SSD can be soldered to the motherboard, or on a plug-in module. A plug-in module might be of an Apple-specific type. On some Macs for which SSDs and Fusion Drives were an option, Apple only included the part of the main board needed to plug in a SSD if you ordered a SSD or a Fusion Drive from the factory. If you bought a machine with just a HDD, there was no place to plug in a SSD later.

If the Fusion Drive is operating correctly, the SSD will not show up as a separate drive in the Finder.

macOS creates a logical drive, in software, whose contents are spread across the SSD and the HDD. The SSD is not merely a cache for the hard drive. Some things may be stored only on the SSD, others only on the HDD, and should you break the connection, or remove either physical half, you destroy the integrity of the Fusion Drive. It's completely up to macOS where to put things – there are no manual controls to force macOS to put something on the SSD or to put it on the HDD.

The OP’s tag line says that they have a 27” iMac running Sequoia. If this is accurate, that would imply that it is a 2019 or 2020 model.

If it is one of those models, and it had a 1 TB Fusion Drive, that drive would have a very small amount of SSD storage (32 GB or less). An external USB-C (USB 3.1 Gen 2) / NVMe SSD would likely be faster, overall. (A Thunderbolt 3 / NVMe SSD would be fastest in absolute terms, but also significantly more expensive.)

Servant of Cats wrote:

The OP’s tag line says that they have a 27” iMac running Sequoia. If this is accurate, that would imply that it is a 2019 or 2020 model.

I believe the "tag line" comes from the options the OP manually selects (or leaves as default) when they create their post; and that it's not an ID of the system they are posting from. @uras146 did ask in his/her next post about a late 2013 27" iMac, hence the reason @den.thed and I responded about a 2013 iMac.

It certainly would help if the OP clearly identifies the iMac he/she is asking about.

Regardless, using an external boot drive is way more desirable than opening a slim iMac. Even the original aluminum iMacs (with the 1" deep body) were not a joy to open; I did one of those once and was able to replace the CMOS battery without completely removing the display panel, but even that operation was a nailbiter.

uras146 wrote:

Samsung evo 860 1tb SATA

I would not bother putting a Samsung 860 inside an iMac. The 860 is a discontinued drive. In its day it was considered a decent SSD but not compared to what's available today. You said you got it free ... does that also mean used?

As already suggested multiple times, use an external drive (SSD) connected to your iMac; don't try putting one inside. Here are two enclosures to consider for your Samsung 860:

OWC Mercury Elite Pro mini

OWC Express

What SSD...?

(make and model #)

Instead of operating on the 2013 iMac and install that Samsung SSD internally. I would put that Samsung SSD in an external 3.0 drive enclosure and run it externally.