Can I just leave the PCIE flash storage uninstalled but upgrade the internal HDD to a SSD on 2015 Fusion Drive iMac?

You do realize that to remove the blade NVMe SSD you must remove just about every internal component in order to remove the Logic Board since that SSD is installed on the back of the Logic Board? That is a very tricky procedure even for an experienced tech with lots of opportunities to accidentally damage something.

If you are going to remove the internal blade NVMe SSD, then I agree with @Allan Jones that it makes sense to replace the blade SSD with an upgrade for the much faster performance.

If you do not want to remove the blade style SSD due to the complex nature of the upgrade, then replacing the internal hard drive with a SATA III SSD is fine. If the internal blade style SSD is too small to be used for storing data (make sure to erase the small blade SSD first so it is empty since being part of a broken Fusion Drive setup may make things more complicated), then you can easily ignore it, or you can have macOS automatically prevent it from mounting during boot by using the information in the following article written by a respected forum contributor:

Prevent a volume from mounting at startup - Apple Community

You should definitely check out the OWC SSD upgrade video to see exactly what is involved in removing the blade style SSD. The video makes it look much easier than it really is.

If you have the Late-2015 model.....are you already installed the macOS 12.x Monterey upgrade? If not, then make sure to install macOS 12.x Monterey now while you still have the original Apple drives installed internally since the macOS Monterey installer will refuse to update the system firmware otherwise which prevents installation of macOS Monterey. Once the macOS Monterey installer updates the system firmware, then you can reinstall Monterey even with third party internal drives installed. If your iMac currently is not able to boot because of a broken Fusion Drive, then you could just install macOS Monterey to an external drive in order to have the Monterey installer update the system firmware....I believe it will use the hidden EFI partition on the internal SSD....if not you could erase the internal SSD to make it work.

I believe it would – but opening up the Mac to replace the existing drives with a new drive, and then sealing up the Mac again, would be a project for a repair shop. You might have to pay for labor at rates of ~$100 USD per hour in addition to the cost of the new parts.

None of the 2015 iMacs have Thunderbolt 3 or USB-C (USB 3.1 Gen 2). On the other hand, all of them have four USB-A (USB 3.0) ports. An external USB 3.* SSD would be nearly as fast as a 2.5" internal SATA III SSD. But you wouldn't have to pay a repair shop to install a USB 3.* SSD. You'd just

• Plug it in (with the help of an appropriate USB-C (USB) to USB-A (USB) adapter, if necessary)
• Erase and format it using Disk Utility
• Use Carbon Copy Cloner or SuperDuper! to clone the contents of your internal Fusion Drive onto it – or maybe, do a "clean install" of macOS on the external SSD, boot from the external SSD, and run Migration Assistant with the place to migrate from being the internal Fusion Drive
• Set the external SSD to be your Startup Disk
• Boot up using it, and not look back

You could then ignore the Fusion Drive, use it as a destination for clone backups, whatever. As long as that Fusion Drive was not failing, and throwing errors that caused problems for the Mac, there should be no issue doing this.

And when it comes time to upgrade your Mac, it will hopefully be trivial to unplug your main startup drive (your new external SSD), plug it into the new Mac, and use it as the place to migrate from, for that Mac's Migration Assistant.

Yes that will work.

For Upgrade options and a Video, see > https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019

For other DIY sites and Videos, you'll have to troll iFixit and YouTube.

If you must open the computer, it makes far more sense to upgrade the existing NVME SSD for far greater speed in a 2015 iMac:

https://eshop.macsales.com/item/OWC/SP4P1T1MA05K/

That will do transfer speeds well over 2000MB/sec. Changing the SATA hard drive for an SATA 6GB SSD internally will do 500-600MB/sec. Changing the SSD is probably easier than changing the HDD and does not require a drive carrier or temperature sensor cable.

Also, a healthy original Fusion drive is probably faster than the 600MB/sec you would see if you changed out the HDD component of Fusion. In a 27-inch iMac with a healthy Fusion drive, write speeds are 600-900MB/sec and reads over 1300MB/sec.

The least expensive incarnation of the external USB SSD option will do ~400MB/sec. Not at all bad considering it does not require opening the computer, something Apple did not incude in the design of 2012 and later iMacs.

Allan Jones wrote:

Also, a healthy original Fusion drive is probably faster than the 600MB/sec you would see if you changed out the HDD component of Fusion. In a 27-inch iMac with a healthy Fusion drive, write speeds are 600-900MB/sec and reads over 1300MB/sec.

Depends on the Fusion Drive.

Apple released four iMacs ion 2015: one in Mid 2015, three in Late 2015. The OP has not told us which one he has or how large its Fusion Drive is.

A Mid-2015 Fusion Drive would have had 128 GB of SSD storage, whether the hard drive was a 1 TB one or a 3 TB one. In Late 2015 Macs, Apple reduced the amount of SSD storage included with a 1 TB Fusion Drive from 128 GB to a measly 24 GB. Yes, it's prime real estate – but on those Fusion Drives, there's not nearly enough of it.

9to5Mac (October 13, 2015) – Apple slims 1 TB Fusion Drive down to a measly 24 GB of flash storage

Depends on the Fusion Drive.

Apple released four iMacs ion 2015: one in Mid 2015, three in Late 2015. The OP has not told us which one he has or how large its Fusion Drive is.

Exactly. We don't know which Fusion config our OP has.

FYI, I'm seeing evidence that suggests the speeds of the Fusions may depend more on the models' screen size and the HDD component that the screen size forces. The Fusion HDD component in most thin-case 27-inch models is a proper desktop-class 3.5-inch 7200prm SATA 6GB mech drive. In Etrecheck reports posted here, those will do Writes up to 900MB/sec and Reads up to 2000MB/sec.

The 21.5-inch models have no room for a proper desktop hard drive so the HDD component is a 2.5-inch 5400rpm SATA 3GB. Writes tend to be 500-700MB/sec and Reads are seldom over 1200-1300MB/sec.

When the same drives are used in mech-only config, collected scores for the 3.5-inch 7200rpmSATA 6GB drives are as high as 170MB/sec. The highest scores I have for the 2.5 5400rpm SATA 3GB in non-Fusion config are 80MB/sec. That must have a bearing on how the same hard drives affect speed in a Fusion setup.

To get a better picture and based on your useful comment, I will add fields to my collection of Fusion drive data to include SSD size and better tracking of the sub-models. Thanks!

AJ

Depends on file sizes, once you exceed the size of the SSD part it slows greatly...