Standard RAM: 8 GB Maximum RAM: 32 GB* · RAM Type: PC4-19200 DDR4* Min. RAM Speed: 2400 MHz · Details: *8 GB of RAM is standard and reported as onboard with an optional upgrade to 16 GB of RAM at the time of purchase for an extra US$200. However, in a pleasant surprise, this memory actually is not onboard and can be replaced or upgraded after the initial purchase of the Mac, it's just unnecessarily complicated. This model can "unofficially" support 32 GB of RAM. · Here's how to install it: · iMac Intel 21.5" Retina 4K Display (2017) RAM Replacement

Great question! If you enter this exact question into the website www.google.com , you will get a link to Macsales.com/OWC . They have great memory but it's not the cheapest. The second link is https://everymac.com/systems/apple/imac/imac-aluminum-tapered-edge-faq/how-to-upgrade-imac-ram-aluminum-2012.html It has the following specifications (you didn't say which 2017 21.5" iMac you have) Mid-2017 (21.5") 2133 MHz PC4-17000 DDR4 2 16 GB Mid-2017 (21.5", 4K) 2400 MHz PC4-19200 DDR4 2 32 GB Also: It certainly is unfortunate that Apple decided to make it so difficult, or even impossible, to upgrade the RAM on the 21.5-Inch models. In particular, using adhesive to attach the display to the chassis -- rather than magnets like previous Aluminum iMac models -- is a design decision that is inelegant at best. As Apple does not consider the RAM in the applicable 21.5-Inch iMac models to be a "customer installable part," EveryMac.com cannot recommend that users perform the upgrade themselves. They link to an install video: https://youtu.be/MuC6a8j8XbQ You can also use Google to search the internet for things other than Mac information!

The new 27" iMac's do have the same access door in the back as the last model, as seen here: iMac Intel 27" Retina 5K Display RAM Replacement. · Upgrading the PCIe SSD requires almost completely tearing down the system as seen here: iMac Intel 27" Retina 5K Display SSD Replacement. So far we don't know if you buy a HDD model only if the PCIe SSD slot and the sundry parts are even present as Apple has in the past has not populated the logic board (Update: All of the 27” systems to 2019 have the SSD slot) This is the case in the 21.5" model as you can see here in the IFIXIT teardown:: iMac Intel 21.5" Retina 4K Display 2017 Teardown · So I would recommend you get the PCIe SSD based system and I also recommend you get the largest SSD you can get as Apple is still the only source for their custom SSD. · Then if you want to add more storage (HDD or SSD). You can then add in the SATA drive later. · Just keep in mind these 'Thin Series' systems are a bear to open so make sure you follow the IFIXIT guides to the letter and use the correct tools as you can easily damage the display in the process of taking the display assembly off which would be a very expensive opps! · Update (06/19/2017) · The term 'Hard Drive' is passé, instead we use terms HDD or HD for the older traditional spinning disk drives or SSD for the solid state drives. If you don't know which then just use the term 'drive'. · So yes, its best to get the SSD only model or the Fusion Drive model which has both a HDD and SSD fused together as one logical volume (two drives seen as one). · As to the PCIe SSD its self, Apple does not sell it as spare (only as an exchange of the same size) and the parts houses which buy compete systems and tear them apart for the parts charge premium prices! So get the largest PCIe SSD you can afford now. · As I stated if you got the PCIe SSD only system you would still have a spare SATA drive bay (3.5" is what we expect here until IFIXIT does the teardown to verify) which will allow you to add in your own drive which could be a HDD or SSD. But! It's still a bit of work as you'll need to get the proper SATA/Power cable harness from Apple and as you are putting your own drive in you'll need this: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later as unlike the custom Apple HD drives you need to add in a thermal sensor for the system to work properly. · If you get a Fusion Drive'd model you will have a dual drive system so you would need to swap out the SATA drive to a larger one if that was your direction later on. · Frankly, Apple is really making it harder to do any drive upgrades these days with the 'Thin Series' iMac's. If you can, get what you want now for drive storage. · Given the risks of damage and the amount of work to even get into the system it's not something I recommend for someone who has not opened one of these newer 'Thin Series' systems before.

Apple offers four 21.5” A1418 systems! Three of which have 4K display and one that carries over the older display. · So lets look at the low end model: iMac 21.5" 2.3 GHz i5 (Mid-2017) Here we can see the max this system can support is 16 GB. · Now lets look at the next one up which has a 4K display: iMac 21.5" 3.0 GHz i5, 4K, (Mid-2017). And the next one: iMac 21.5" 3.4 GHz i5, 4K (Mid-2017) Both of which support 32 GB of RAM. · So why is this the case? It turns out the low end is using the same logic board of the older Late 2015 models with just an upgraded CPU which is why its RAM is limited. The newer 4K models have a very different logic board as these model use a newer CPU socket and PCH logic which is why these support the higher RAM! · Now if you are planing on upgrading the RAM on your own. Be very careful taking the display off as you can damage it quite easily it you don’t use the proper tools and techniques as outlined in the IFIXIT guides.