imac 2019 27in 5k. upgrade question

Ive upgraded at least twenty 2019 iMacs in the past two years, including retail m2 SSDs, Apple NVMe SSDs, SATA SSDs, RAM (in 4k models), button-battery replacements and blowing out the internal dust

Phil0124 is spot on, albeit a bit dramatic

first i always prefer to boot from internal SSD

second, Thunderbolt/NVMe SSD will deliver faster IO than the Apple SSD (NVMe)

OWC puts out the solid DIY videos (how i learned, step by step)

original Apple SSD (NVMe): about 2100MBs write / 2500 read

orininal 27” 5K Apple SATA HDD: about 180MBs write/read

original 21” 4K Apple SATA HDD: about 80MBs write/read

SATA SSD upgrade: about 500MBs write/read

Thunderbolt external M.2 SSD: about 2600MBs read/write

i recommend ACASIS TBU405 Pro enclosure and Samsung 990 Pro SSD

i only use internal OEM Apple SSD (Samsung SSPOLARIS) any more (2TB are around $300, 100% health)

i favor Samsung 860 PRO or 870 EVO SSD for SATA

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the 2019 5k 27” iMacs are my favorite work machine for several reasons:

they run Mojave to Sequoia (run Photoshop cs6)

the 5k IPS LCD is amazing

the ram is easy upgrade to 128GB

easy to work on — lots of parts available…

Using an external boot drive is the simplest & easiest way to upgrade your iMac. Opening your iMac to replace or upgrade internal parts is a delicate and potentially risky operation. (Risky to both you and the iMac.)

Consider the OWC Express 1M2 80G. Supports Thunderbolt 3, matching your 2019 iMac TB ports. You can get it as a bare enclosure and put in your own NVME M.2 blade or buy it with an OWC Aura Ultra IV PCIe 4.0 NVMe M.2 2280 already installed.

I have the same iMac. I use a WD SN850X 1TB NVME drive in an Acasis TB enclosure. I have Monterey on the Internal and Sequoia on the external which lets me switch back and forth as needed. Works great.

Unless you thrive on risk, I would avoid any encouragement to open that 2019 iMac as a DYI project. Because someone else has done this n-tuple times — does not make it easier, nor reduce your risk. One mistake and you have a paper weight.

A modern SSD that is supported in macOS Sequoia (last macOS for 2019 iMacs) is likely to be faster than the internal drive on the iMac when it is a USB-C 3.2 Gen 2 drive (and rated cable) connected to a Thunderbolt 3 port.

There has been a substantial increase in SSD drive pricing over the last year. My OWC 2TB Envoy Ultra TB5 SSD that I purchased in Sept 2025 for $399 is now $699 — a 75% price increase.

Unless you are experienced at disassembling the iMac ands removing the glass screen by cutting through the glue and the gasket and very carefully disconnecting the cables to the screen and unscrewing a bunch of stuff to get access to the SSD, I would see if booting from an external drive is good enough for your needs first.

You'll need tp get the special iMac screen glue strips to re-adhere the screen once you are done with the replacement. The process is definitely not for the faint of heart.

Samsung T7 or T9 are good options.

Or if you want really high end options, OWC.

click here ➜ Samsung T9 SSD 2000MB/s - Amazon.com

and here ➜ SAMSUNG T7 Portable SSD, 2TB External Solid State Drive, Speeds Up to 1,050MB/s - Amazon.com

and here ➜ OWC Mercury Electra 6G 2TB 2.5-Inch SATA III Internal SSD, 7mm Solid State Drive with 6 Gb/s Speed - Amazon.com

-g wrote:

Phil0124 is spot on, albeit a bit dramatic

Ha ha, thanks.

tbirdvet >> TB4 and TB5 enclosures that will not work with my 2019 27" iMac since it only has TB3 ports

I believe it is Thunderbolt 5 enclosures that you are talking about

they "worked" for me on 2019 TB3 (and TB4 Macs), but at slower USB speed (900 MBs)

the ACASIS TBU 405 Pro is a 40Gbps TB4 enclosure

it delivers 2600MBs WRITE and 2700MBs READ via the 2019 (and 2017) Thunderbolt 3 ports

as seen in my screenshot below

I state this with certainty

when they released the ACASIS TB501 80Gbps TB5 enclosure

their product details noted it was "compatible" with TB3-4

I bought one thinking TB5 is better than TB4 and I would still get the TB3 speed, but it only ran at 900 MBs read/write speed through my 2019 TB3

that is a known hardware issue -- 900MBs is the max speed you will get out of a Mac TB3 and/or TB4 port from a TB5 SSD drive enclosure

I just noticed ACASIS also notes this TB501 as "compatible" with USB4 (whatever they mean by that) if it maxes at 900MBs or the full 80Gbps speed via USB4...

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THE SHORT ANSWER here is don't buy an ACASIS TB5 enclosure for use on TB3-4

only buy the TB5 enclosure if your machine is actual TB5 hardware

I imagine all thunderbolt 5 enclosures will exhibit the 900MBs speed on all Thunderbolt 3-4 machines, but I sent mine back and bought another TBU 405 Pro because a TB5 machine is not in my near future

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if anyone is getting anything near 2500MBs read/write from a thunderbolt 5 (or USB4) SSD enclosure -- connected to a Mac TB3-4 port -- I would be interested in the details...

doulberis wrote:

Would it be faster and more convenient to install there the macOS? even with open legacy to get newer version?

Your 2019 iMac supports Sequoia. Sequoia will still be one of the "most recent three" for a year after macOS 27 (Golden Gate) comes out.

We cannot discuss hacks ("open legacy") on these boards, and seeing as how macOS 27 (Golden Gate) will be released for Apple Silicon Macs only, no hack is going to let your Intel-based 2019 iMac run it.

I am a medical doctor so I use it mostlz for work, emails, browsing and some basic photoshop. Please tell me if the endauver does worth or not for an old machine..for older patients at least, we rather avoid operations... Thanks in advance, Best regards Michael

Using an external USB-C (USB 3.1 Gen 2) or Thunderbolt 3 SSD as a startup drive, sure.

Hacking macOS – stay away.

doulberis wrote:

Dear friends

is it wise and safe also from temperature perspective to format my iMac and use the external nvme for first drive for main usage of MacOs? Is it safe? Fast will be, I totally understand

You don't need to erase the internal drive in your Mac to set up, and start up from, an external SSD. I do not think erasing the internal drive would buy you anything in terms of temperature control. (It also might be useful to have a bootable system on the internal drive in case anything were ever to happen to the external one.)

As for other temperature-related questions, I'll leave those to others.

Please be careful when selecting a TB enclosure. I have some TB4 and TB5 enclosures that will not work with my 2019 27" iMac since it only has TB3 ports but work great on my Silicon Macs..

have you timed Samsung T9 on 2019 hardware -- and seen a 2000MB/s speed result? -- that's a USBc drive correct?

Thunderbolt is the king of externals -- providing the maximum read/write speeds 2019 hardware can deliver

granted, not for everyone...

>> have you timed Samsung T9 on 2019 hardware -- and seen a 2000MB/s speed result? -- that's a USBc drive correct?

>> I have not. It's the published specs.

I recall USB-C is limited to 900MBs on 2019 Mac hardware

AJA Thunderbolt speed test (in addition, ACASIS allows easy swapping M.2 SSDs, plug and play):

-g wrote:

>>MartinR the OWC Express 1M2 80G. Supports Thunderbolt 3

have you clocked one of these on 2019 Thunderbolt 3?

That one supports Thunderbolt 3 – on Macs only – but it seems to be a fallback mode. OWC's own advertising says that the maximum transfer rate depends on the host interface.

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• USB4 80Gb/s or Thunderbolt 5 host – Over 6000MB/s
• USB4 40Gb/s host – Over 3800MB/s
• Thunderbolt 4 host – Up to 3800MB/s
• Thunderbolt 3 host (Mac only) – Up to 2800MB/s

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Note the "up to". Say you are on a Mac with Thunderbolt 5 ports, and you are copying enormous video files to a SSD in this enclosure. If you fill up the SSD's internal write buffer, and it starts accepting data at a slower rate as a result, you might not get the transfer rate that Thunderbolt 5 could support.

Between the Thunderbolt 5 capability (which the OP wouldn't be able to use right away), and the recent spikes in SSD prices, one of these is pretty expensive: $494.99 for a 1 TB SSD!

Some TB4 enclosures will not work. I have a Satechi TB4 enclosure that has great speeds on my Silicone Macs but no go on the 2019 27" iMac. It has to do with the chip level in the enclosure

>>MartinR the OWC Express 1M2 80G. Supports Thunderbolt 3

have you clocked one of these on 2019 Thunderbolt 3?

I am curious — Detail from Owc:

“Express 1M2 80G utilizes the latest USB4 technology and a USB-C connector”

there are some interesting differences between usb4 vs tb3 (outside of 2019 hardware)

I never been an easy simple operator when it comes to speed — more ram — fastest drives — the machine and macOS that gets the job done

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when you can’t see your deadline for the blood in your eyes then you will understand