Upgraded Late 2012 iMac Fusion Drive - Now very sluggish

[uptownnyc]

The hard drive of my 27" Late 2012 iMac with a 3tb Fusion Drive died recently. This was the second drive of the machine -- the first had been replaced as part of a repair program.

I swapped it out with a 4tb 7200 RPM OWC DIY kit. Replacement was easy, and it's all working --- but I'm noticing what seems like excessive sluggishness with the new drive. I realize I've switched from an SSD/HDD hybrid to all spinning platters, but it sounds like my drive is constantly churning even when the machine has no workload.

I realize I'm working with hardware that's past its expected lifespan, but I'm wondering if there's anything else I can do to improve its performance with the new drive, or if what's sold as part of that kit just isn't expected to perform close to what the fusion drive had provided.

 

[DeltaMac]

Just to clarify here: The spinning hard drive that you replaced was NOT a hybrid device. The Fusion drive is two separate devices. A spinning hard drive, which you replaced, and the existing SSD, which remains as original, unless you replaced THAT, too!
The fusion drive, as Apple uses it, is not hardware, but software, creating a virtual drive that uses both devices.

Your symptoms may indicate that you are not using the SSD at all, and you can re-join the drives get the performance that you are expecting.
Did you rejoin the new hard drive with the existing SSD, so that the two devices still function as a fusion drive?
Here's the process to make the fusion drive in your terminal -https://www.techrepublic.com/article/pro-tip-how-to-create-and-disable-a-fusion-drive/

 

[Juicy Box]

Just to clarify here: The spinning hard drive that you replaced was NOT a hybrid device. The Fusion drive is two separate devices. A spinning hard drive, which you replaced, and the existing SSD, which remains as original, unless you replaced THAT, too!
The fusion drive, as Apple uses it, is not hardware, but software, creating a virtual drive that uses both devices.

Your symptoms may indicate that you are not using the SSD at all, and you can re-join the drives get the performance that you are expecting.
Did you rejoin the new hard drive with the existing SSD, so that the two devices still function as a fusion drive?
Here's the process to make the fusion drive in your terminal -https://www.techrepublic.com/article/pro-tip-how-to-create-and-disable-a-fusion-drive/

OP: Based on everything said, I would start with this post above.

You are most likely going full HDD, which would be noticeably slower than the Fusion Drive.

If you get a chance, download Blackmagic disk test to get some actual read write speeds for us. It is free in the App Store.

 

[uptownnyc]

Did you rejoin the new hard drive with the existing SSD, so that the two devices still function as a fusion drive?

I didn't. It sounds like that's my problem. Thanks for the link. Is there any concern regarding data-integrity with my rejoining the drives now that I've been operating with just the spinning HDD for awhile? I see the following in my Disk Utility (Vertex is the name of the new 4tb drive).

 

[Fishrrman]

The platter-based drive -- running UN-fused -- is never going to be "fast enough to be satisfying".
You need to "re-fuse" it with the internal SSD.

 

[uptownnyc]

If you get a chance, download Blackmagic disk test to get some actual read write speeds for us.

 

[rbart]

Your new HDD seems very very slow and you have made a mistake when setting it up: it's not configured to be used as fusion drive. You just use the very slow HDD.

 

[jseq]

I also have a 2012 iMac and the best advice i can give you is to run the OS on an external ssd with USB3 enclosure. The difference in performance is enormous and inexpensive!
Speeds around 450MB/s.

 

[DeltaMac]

It's been a while since I have done the "re-fuse" to recreate a fusion drive -- but, I am pretty sure that doing that will erase both drives.
(It's probably important to be sure to have a full backup before you start that re-fuse process...)

 

[uptownnyc]

It's been a while since I have done the "re-fuse" to recreate a fusion drive -- but, I am pretty sure that doing that will erase both drives.
(It's probably important to be sure to have a full backup before you start that re-fuse process...)

Cool. Thanks for pointing out my mistake. I'll make sure I've got a TimeMachine backup and give this a try.

 

[uptownnyc]

Finally got around to resolving this. I tried to reset the drive to fusion mode, and it alerted me to S.M.A.R.T errors while trying to do the OS re-install. Replaced the drive a second time, and now I'm back to suitable performance.

 

[Fishrrman]

If the OP had just plugged in a USB3 external SSD, and set that up to be the boot drive, he'd be getting reads of 430MBps and writes in the 330-350MPps range... (sigh).

There's a reason I've been suggesting this method for quite a few years now, for improving performance on [now getting older] USB3 Macs...

 

[Azrael9]

I bought an external Samsung SSD and it's speed surpassed the original SSD 128 gig internal part of the 'Fusion' drive.

Inexpensive.

Probably faster and bigger SSDs out there now for relatively little coin.

Azrael.