How to Add More Storage to Your iMac in 2025
Your iMac is too good to be held back by a slow hard drive. And yet, Apple shipped millions of them with spinning HDDs right up until 2019 — drives that were already outdated the day they left the factory. If yours is still running on a hard drive, or even a modest Fusion Drive, there's a reasonable chance you've been watching a lot of spinning beach balls and assuming it's just "how Macs get with age."
It isn't. The Mac is fine. The drive isn't.
This guide covers every storage upgrade path available for iMac owners in 2025, from a straightforward internal SSD swap on older 27-inch models to the best external options for newer M-series machines. No warranty voiding, no software hacks — just a clear explanation of what's possible and what to buy.
First: Which iMac Do You Have?
The right upgrade depends entirely on your model. Here's the quick breakdown:
- iMac 27-inch (2012–2019): Internal HDD or Fusion Drive that can be replaced with a proper SSD. This is the upgrade that makes the biggest difference — and it's very doable as a DIY project.
- iMac 24-inch (M1, M3) and iMac 27-inch (2020, Intel): Internal storage is soldered. You cannot upgrade the internal SSD. Your only option is fast external storage, and there are some excellent ones.
- iMac Pro (2017): Internal storage is soldered and requires specialist tools and significant expertise. External storage is the practical path here too.
Not sure which model you have? Click the Apple menu, choose "About This Mac", and check the year. Or look at the back of the stand — the model number is printed there.
Path 1: Internal SSD Upgrade (iMac 27-inch, 2012–2019)
If you have a 27-inch iMac from 2012 through to 2019 that shipped with a spinning hard drive or a Fusion Drive, replacing it with an SSD is the single most impactful upgrade you can make to that machine. We're talking boot times dropping from 60+ seconds to under 15. Applications launching instantly. The whole computer feeling like new.
The reason this works — and the reason it requires a specific kit rather than just any SSD — is the iMac's thermal management system. Apple built these iMacs to monitor drive temperature via a proprietary sensor cable. If you just swap in a third-party drive without the right adapter, the fans run at full speed permanently. It's loud. It's annoying. The fix is an in-line digital thermal sensor, which is included in both kits below.
Option A: OWC Mercury Extreme 6G SSD Upgrade Kit
The OWC DIY Bundle with Mercury Extreme Pro 6G SSD is the complete package for a 2012–2019 iMac 27-inch. Everything you need is in the box: the SSD itself (available in 240GB, 480GB, 1TB, 2TB, and 4TB), the AdaptaDrive bracket to fit a 2.5-inch SSD into the 3.5-inch bay, the in-line digital thermal sensor, suction cups for removing the display, Torx T10 and T8 drivers, adhesive tape for resealing the screen, a pry tool, and a microfiber cloth. You won't need to buy anything else.
The OWC Mercury Extreme delivers up to 550 MB/s sequential read and 530 MB/s write — over 10x faster than a typical 5400 RPM HDD. It uses SATA 6Gb/s and connects directly to the iMac's internal SATA bus, so there's no bottleneck. OWC backs it with a warranty, and the kit includes access to free installation video guides that walk you through the full process.
The installation itself involves removing the display with suction cups, disconnecting a few cables, swapping the drive, and resealing with the included adhesive tape. It's not complicated, but it does require patience and care. Watch the video before you start — OWC's guides are thorough.
Option B: Flexx DIY Bundle with Samsung 870 EVO
The Flexx DIY Bundle with Samsung 870 EVO is our own-brand kit using the Samsung 870 EVO — one of the most proven and reliable SSDs ever made. Same compatibility (2012–2019 27-inch iMac), same included tools, same thermal sensor. Available in 250GB, 500GB, 1TB, 2TB, and 4TB. The Samsung 870 EVO delivers up to 560 MB/s read and 530 MB/s write, and carries a 5-year Samsung warranty.
If you already trust Samsung storage — and there are good reasons to — this is the kit to go for. It's slightly more affordable than the OWC option at comparable capacities and comes with everything you need to complete the upgrade.
Which to choose? Both are excellent. The OWC kit uses OWC's own SSD with RAISE data protection technology. The Samsung kit uses the 870 EVO, which has an outstanding long-term reliability record. Either way, you'll get the same night-and-day improvement over the original hard drive.
Path 2: External SSD (iMac M1, M3, 2020 Intel, and iMac Pro)
If your iMac has soldered storage — which includes all M-series iMacs and the 2020 Intel models — you can't touch the internal SSD. What you can do is add fast external storage that offloads your working files, media libraries, scratch disks, or overflow, so the internal SSD handles the OS and apps while the external drive handles the heavy lifting.
The key here is connection speed. A USB 3 drive is fine for backups and file archives. For anything you're actively working from — video footage, Lightroom libraries, Logic Pro projects — you want Thunderbolt or USB4.
For Most Users: OWC Envoy Pro FX
The OWC Envoy Pro FX is the external drive we'd recommend for the majority of iMac owners. It connects via Thunderbolt 3 or USB-C, reaches speeds up to 2800 MB/s, is IP67 rated for dust and water resistance, and is completely bus-powered — one cable to the back of your iMac and it's running. Available in 480GB, 1TB, 2TB, and 4TB, with a 3-year OWC warranty.
At 2800 MB/s, it's faster than the internal SSD in many older iMac configurations. If you're working with 4K video, large photo libraries, or audio projects, it handles all of it without any slowdown. The rugged housing means it's equally happy sitting on a desk permanently or being taken out with a MacBook.
For Maximum Speed: OWC Express 1M2 USB4
If you have a newer iMac with USB4 ports — the M3 iMac in particular — the OWC Express 1M2 USB4 gives you speeds up to 3151 MB/s from a palm-sized enclosure. It's the fastest bus-powered single drive available, and it uses true USB4 40Gb/s — not the watered-down versions many cheaper enclosures claim. Available from 1TB to 8TB.
For Future-Proofing: OWC Express 1M2 80G (Thunderbolt 5)
If you've just bought or are planning to buy a new iMac when Thunderbolt 5 models arrive, the OWC Express 1M2 80G is built for those 80Gb/s connections. Speeds up to 6000 MB/s when the host can support it. It works on current TB4 and USB4 machines too, just at lower speeds. The enclosure starts from £225 without SSD — you fit your own M.2 NVMe.
What About the iMac 21.5-inch (2012–2019)?
The 21.5-inch Intel iMacs are a different story. The internal storage is significantly harder to access than the 27-inch — many configurations require removing the logic board entirely, which puts them firmly in "professional repair shop" territory rather than a DIY project. If you have a 21.5-inch with a slow HDD, the practical recommendation is fast external storage rather than an internal swap. The Envoy Pro FX connected via Thunderbolt is a cleaner solution with less risk.
Does This Void the Warranty?
For any iMac still under AppleCare, yes — opening the machine and replacing internal components can affect your coverage. If your iMac is out of warranty (which any 2012–2019 model certainly is), there's nothing to lose. The DIY kits from OWC include everything needed to do the job properly without damaging the machine. Thousands of iMac owners have done this upgrade successfully.
If you're uncomfortable opening the machine yourself, the alternative is simply going the external SSD route — it delivers enormous real-world benefits with zero risk and takes about 30 seconds to set up.
Free Shipping to the UK and Ireland
All upgrade kits and external drives ship free to the UK in approximately 2 working days. Orders to Ireland over £100 qualify for free shipping too. If you're not sure which path is right for your specific iMac model, get in touch — we're happy to point you in the right direction before you buy.