Recommendation for macOS on 2015 iMac?

A neighbor/friend of mine has an ‘older’ iMac (Late-2015, Retina 4K 21.5"). He’s had High Sierra on it for quite some time. His usage is quite modest: mostly email, Safari, Photos, and MS Word. For several/many months, the MS updater has been nagging him that the Office software couldn’t be updated because his version of macOS was too old to support the latest Office release. A couple of months ago, he was induced by Apple’s Software Updater to update to Monterey: “An update is available. Click here to install…”. So he did.

Since then, the iMac has become sluggish and nigh unusable. He called me for help. I found that the iMac now has Monterey 12.6.3, and the internal 1TB hard drive has been converted to APFS. Presumably, the Monterey installer took the liberty of converting the hard drive from HFS+ to APFS. It appears that’s the root cause of the poor performance. According to the esteemed expert Joe Kissell (author of Take Control of Monterey), the performance of APFS on a mechanical hard drive is “terrible”, and “almost completely unusable”. Joe wrote "if you have a Mac with a plain hard drive, you should think twice about installing Monterey". It’s a shame that Apple’s Software Updater didn’t heed Joe’s advice.

At this point it seems the options are:

  • replace the internal HD with an SD. Seems like a waste of $$ on a 8-year old iMac.
  • wipe the internal drive and reinstall High Sierra from scratch. (fortunately he has Time Machine backups).
  • procure an external SSD, connect it with USB3, and use it as the boot drive (for Monterey).

I’d appreciate advice or feedback on these options, or any others that might be viable.
(Buying a new iMac is not on the table at this time).


I’d say if he is willing to spend money you could go the route of external SSD interfaced through the 2015 iMac’s TB2. That will give him a fast boot drive (i.e. not just 5Gbps over the 2015 iMac’s USB 3.0 or using an SSD with SATA interface). The cheapest I’m aware of to interface 2TB of decent NVMe flash over TB3/2 will run ~$220. Details below.

If he is not willing to spend that amount on his old iMac, I’d say your only reasonable option is wipe, reformat as HFS+, install older macOS from scratch, and use use MA to restore from TM (as you indicate in option #2). IIRC, 10.14 Mojave would be the last version to support 32 bit apps, while 10.13 HS introduced APFS (did not outright require it though IIRC).

I’m afraid I got my genders mixed up. You’ll also need these for an extra $39

(Yeah, it ain’t pretty.)

Thanks for the advice, and the links to all the bits and pieces.
You’re right - it ain’t pretty.

I think I’d be inclined to go with an external SSD with USB3. Simpler, cheaper. Certainly not as fast as using an NVMe and TB, but perhaps comparable to the speed with the internal SATA (which is what he’s used to). No doubt it would boot faster using the TB approach, but once the iMac is up and running, he might not notice any difference for day-to-day use: opening/saving Word docs, doing email, etc. That’s entirely speculation - I have no experience booting from external SSDs.

I have a Samsung T5 handy - I could give it a try.


Another option, after reinstalling High Sierra from Time Machine, is to “upgrade” to Mojave, if you can locate an installer.
My MS Office 365 seems to be happy with Mojave on an iMac. I have numerous (32 bit) apps that won’t run in Monterey or above (for what it is worth, I also have a Macbook Air running Ventura)

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I hear that. The same 2TB stick will work with an inexpensive USB case which you can hook up directly to the iMac’s 5 Gbps USB-A. Total cost is then down to $100. Not nothing, but hard to beat.


FWIW, Microsoft currently updates MS Office 365 only on the three most recent major versions of macOS, i.e., Ventura, Monterey, and Big Sur at this writing.

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I am running an 2017 iMac from an external 1TB SanDisk SSD attached via USB and no one in the office has mentioned any problems.

I have that iMac. Several years ago I looked at the options you outlined and decided busting open the box was a nonstarter for me so, it’s currently running 12.6.3 booting from a Samsung T5 via USB. It’s much faster than the fusion drive that came with the iMac and I’ve had zero problems with day to day operations. I’m using the fusion drive, among others, as a SuperDuper backup. It does take an inordinate amount of time to reboot after a shutdown but that may be due to the ridiculous amount of cruft I have from never doing a clean install since forever. (I actually have AppleWorks docs sprinkled around…) And, I only startup from ‘off’ when the power goes out or I’m installing an update.


In what way? Does it take a long time to start the boot process? Or does it take a long time to boot once the process begins?

If it’s the former, then your PRAM may be pointing at another device (maybe your internal drive?), and only searches the USB bus when that attempt times out. If that’s the case, then you should use the “Startup Disk” preference panel to select your boot device (The T5). See if that helps.

If it’s the latter, then it might be cruft to clean up. But that’s a topic on which I am not expert, so I’ll defer that to someone else.

This is a clip from Apple Support for eligible machines for Ventura.


Identify your iMac

  • iMac (24-inch, M1, 2021)
  • iMac (Retina 5K, 27-inch, 2020)
  • iMac (Retina 5K, 27-inch, 2019)
  • iMac (Retina 4K, 21.5-inch, 2019)
  • iMac (Retina 5K, 27-inch, 2017)
  • iMac (Retina 4K, 21.5-inch, 2017)
  • iMac (21.5-inch, 2017)

This iMac is not listed.

I think that the Ventura message was a miss direction (dare I say a bug?). On my MBP macOS, I’m locked out at Big Sur. A message for a Monterey download kept appearing and I kept ignoring it. This message kept nagging me until Ventura was released.

Takeaway: Always look up the supported Macs when new releases are announced.

Speaking of that, per Apple Support, your late 2015 is eligible for Monterey.

This link is to for your late 2015 iMac system details.


Based on these specs I don’t see much benefit from Thunderbolt.

I’m in a similar situation. I can’t afford a new computer. I’m maxed out at Big Sur and the battery bloats my MBP. I purchased my MBP on eBay in 2017 as a late 2013 model with 16 GB memory, 240 GB SSD drive, and Retina Display on eBay for $1180. Those 6 years for a 4 yr old machine has served me well. Big Sur is next to fall off the support wagon. The used market is full of folks who want the best of everything and the sweet spot for support updates/to used computer prices is about two years after M2 was in play. I have no problem buying a used computer. Just ask questions about care and usage.

Back to the problem.
The solution might be simply to restore macOS with cmd/option/R (a system reset to the original OS) and then the let that install tell you the next OS update. Rise, repeat. I assume the path the Monterey may be blocked by intermittent requirements for a specific download. If the Ventura download appears again, ignore it. (see next)

Go to this link and directly pick an OS, e.g., Monterey.
I can’t remember whether this file is a self-contained install that resets the hard drive or not. I may need some help here.

MacOS library

HTH. Mike

Thanks to all for the feedback and helpful suggestions. In particular to @Simon (for recommendations as to what bits and pieces to get) and to @jaclay (who has faced the same issue with the identical iMac).

I’ve decided to go with an external SSD as boot drive, and have ordered a 1TB Samsung 980 NVMe stick and a UGreen USB3.2 Gen2 enclosure. It’s a bit overkill for this iMac, which only has USB 3.0 interfaces. But eventually, the iMac will go to the Big Recycling Bin in the sky, and the SSD will get redeployed on a faster machine somewhere.

I’m still undecided whether to move to Mojave (retain compatibility with 32-bit apps) or Monterey (still supported by Apple). Ventura is not supported on this old Mac, and even if it were, I wouldn’t go there as I don’t think the owner (an elderly gentleman who’s been using Macs for decades) would be at all happy with Apple’s new approach to system preferences.

Thanks to everyone. I really appreciate the input.

“No results found. Please try another search.”

The Monterey installer isn’t listed on the Apple Download page. Nor does it appear if I search the App Store for it. But this link will finesse the App Store and find the complete installer (12 GB) there.

I came to the same conclusion.

Since you mentioned MS Office, the last version supported on Mojave is the October 2021 release, version 16.54. You can find the installer by scrolling down Microsoft’s “Update history for Office for Mac” page.

Regarding 32bit software, the free Go64 app is a great way to get a list of installed 32bit apps that would need to be upgraded if you go past Mojave.

That one. It’s very strange. I’ll get to the ‘choose account’ screen relatively quickly, but once logged in to my preferred account it seems to take ages to get everything going. The thing that annoys me the most is that the Bluetooth keyboard and mouse disconnect during the process, then reconnect a while later. Obviously something going on with Bluetooth. If I connect via cable, no issues, also if I use my Logitech mouse with a USB dongle no problems. If I log in using my ‘Safety’ account things proceed more smoothly and then I can switch to my main account from the desktop, which seems to go more quickly, but I haven’t developed the habit of going this route. I have two older USB HDs and the Samsung plugged in, and of course the Fusion Drive is hard wired, but unplugging the HDs hasn’t seemed to improve the boot time.

I’m waiting for the M3 iMac or a Mini and Monitor combo that I’d want to use before upgrading so I’m just living with it for now.

Are you using File Vault?

If you are, then when you get to the login screen the first time after a power-on, macOS is not actually running. You’ve got a very minimal operating system that just collects login credentials. After you provide credentials for an account authorized to decrypt the boot volume, the key is unlocked and macOS boots for real.

So the delay you’re seeing may be that internal boot sequence.

Of course, once that’s done, the drive is unlocked and the system is booted. So a subsequent log-out/in won’t have that delay.

Not using File Vault. To me the weirdest thing about this issue is the intermittent Bluetooth connections to mouse and keyboard during the process. Once the system becomes stable everything works fine.

If you were using File Vault, I would say that the Bluetooth behavior is related. That there’s a period of time between when the pre-boot operating system unloads and when the proper macOS device driver loads during which BT doesn’t work.

This might still be the case, but without FV, I’m just guessing. Maybe a hand-off from the boot-ROM’s BT driver to the one in macOS?

That’s not quite right – I’m runnng Mojave on a mid-2012 MBP, and the three major MS apps updated to 16.71 on 28 Dec 2021, after which MS AutoUpdate (which is current as of 14 Mar 2023!) started complaining “MacOS Upgrade Required.” While I can live with that (I have newer versions on a newer machine), I’d really like to find a way to stop AutoUpdate on this machine from popping up several times a day.

It’s true that MS AutoUpdate and a few other apps from Microsoft still are being updated on Mojave (notably Edge, Teams, and OneDrive).

However, it doesn’t add up that version 16.71 of Office was installed on your Mojave machine in December 2021, since 16.71 only was released last month. May I trouble you to double check the versions of Word/Excel/PowerPoint on your Mojave machine? For example, please open Excel and choose “About Microsoft Excel” from the “Excel” menu.

If you do have 16.71 somehow running on your Mojave machine, it would be very interesting to figure out how that happened, since the 16.71 installer and update packages fail on Mojave with the message, “Software Requirement \ This software requires macOS version 11.0 or later.”

Thanks in advance!