iMac 2017 3 TB Fusion Boot Camp slow Performance IO?

The SSD size of 128GB does not lead much room. I have split the SSD into two parts, installed macOS and Windows on the SSD part, and then split the HDD in half, where one half is added to the SSD macOS as a CS part, and the other half is formatted as NTFS to allow Windows non-OS files, including Documents, etc..

If you post the output of diskutil cs list, it is much easier to explain the process and I can provide the exact commands.

Hey Loner T,

I'm almost in the exact same boat as DigitalDigital. I just bought a 2017 27 inch iMac 5k with a custom ordered 3TB Fusion Drive just a couple of days ago and i'm also experiencing slowness on my recently installed Windows 10 Boot Camp partition. I've thought the reason why it runs so slow is because Windows 10 is installed on the spinning disk portion of the iMac and I thought of taking drastic measures by opening up my iMac and replacing the spinning hard disk portion with a 2 TB SSD.

If there's a way where I can increase performance on bootcamp Windows 10 without any need to make changes to the internal hardware, i'd much rather do that instead. I am mainly using Windows 10 simply to play games off of Steam. I have Steam and a handful of games downloaded from it installed on my Windows 10 partition and i've also fully updated the OS to the latest version.

I had no idea you can install Windows 10 into the SSD portion of the iMac and i'd like to do that. Can you give me a step by step rundown on how I can easily do that?

I don't want to touch the existing install of Mac OS X High Sierra on the SSD portion of the iMac and would like to simply add my Windows 10 OS installation to the SSD as well.

By moving the Windows OS to the SSD, would that help improve performance significantly on Windows 10 Bootcamp? Apart from playing games on Windows 10, I'm also planning to do some serious multitasking on Mac OS X and i'm thinking that sticking with a spinning disk drive altogether would still hamper performance on both partitions of the iMac if I were to heavily multitask on both OSs. Would it be best in my situation to just upgrade the internal spinning disk drive inside my iMac to a 2 TB SSD instead?

I hope all of this makes sense. I really want to enjoy the maximum multitasking potential of my iMac and i'd like to do whatever I can to remove any kinds of bottlenecks that could hold my computer back from running at its best performance.

Here's what I get when I run diskutil list:

Here's what I get when I run diskutil cs list:

Also, if upgrading the spinning disk drive to an SSD is the best way to go in my situation, and since I have a fusion drive installed on my iMac with a bootcamp partition of windows 10 installed on it, can I simply clone everything stored onto the spinning hard drive onto the brand new SSD before physically adding it to the iMac? Would everything work the same as usual? Or would I need to create a new fusion drive again?

I could really use your support with this mess. Thanks in advance!

P.S. - Here's the list of specs my 2017 27 inch 5k iMac is currently packing:

4.2 GHz Intel Core i7 Kaby Lake Processor

40 GB 2400 MHz DDR4 RAM

AMD Radeon Pro 580 8 GB Graphics

3 TB Fusion Drive (with 128 GB Internal SSD)

Loner T, can you describe this proces please? I am having the same issue but wouldn't know how to safely use the mentioned command>

There is a method to install on the SSD part, but it requires manual partitioning and installation. The other challenge is to ensure the small 120GB is split judiciously. I run Windows on Macminis with a manually-created partition on the SSD part using the diskutil cs resizeStack command.

You can, but the OS will still be the internal drive designated C:, while the external SSD will be D:.

There is a manual partitioning method to install on the SSD, if that is an option you want to test.

The diskutil cs resizeStack command uses an underlying physical volume to resize. The default for BC Assistant is for it to use the HDD of the Fusion drive. There are two primary drivers for this choice - size of the HDD is much larger than SSD, and reducing the SSD can restrict the growth of macOS and defeat the purpose of a Fusion drive.

However, you can choose a reasonable size for the Windows partition on the internal SSD by manually specifying the SSD as the physical volume to use and still use it for pure Windows OS and put your documents elsewhere - externally or internally. I have done this on both my Minis, one has a 256GB Samsung 840 Pro, the other has a 512GB Samsung 840 Pro, which allows me adequate macOS and Windows disk space.

From macOS Terminal, please post the output of

diskutil list

diskutil cs list

Normally, macOS is installed on the SSD part of the Fusion drive, and the Windows part is installed on the slower HDD part.

Do you have any antivirus software which has real-time protection enabled?

Post back after you have had a chance to look at the running software. One test is to run Windows in Safe Mode with Networking and then checking if it is working a bit better.