How can I utilize a spare Exos 7E8 8TB External HDD to improve late-2015 iMAC performance?

The report that you posted does not indicate that lack of RAM is a problem.

Virtual Memory Information:

Physical RAM: 8 GB

Free RAM: 475 MB

Used RAM: 5.05 GB

Cached files: 2.48 GB

Available RAM: 2.95 GB

Swap Used: 0 B

At the time of the report, your Mac had 475 MB of completely free RAM, and another 2.95 GB of RAM that it could have freed up on a moment's notice by dumping cached files. (Storing cached files in "unused" RAM can help to speed up things, if programs want the contents of those files again, before they want the RAM.)

The report does indicate that your Mac has a 1 TB Fusion Drive with a miserly 24 GB SSD. Apple originally included 128 GB of SSD space with all Fusion Drives, including 1 TB ones. Later they got stingy and only offered 24 – 32 GB of SSD space with the 1 TB ones. That is prime real estate – but there is not nearly enough of it.

You may want to get an external SSD for your Mac and boot off it. Your Mac does not have Thunderbolt 3 or USB4 ports, but even a decent USB-A (USB 3.0) / SATA SSD is likely to beat the performance of your Fusion Drive, as it would be able to deliver high performance consistently, not just for a minuscule fraction of the data on the drive. It would also be possible to connect a USB-C (USB 3.1 Gen 2) / NVMe SSD with the aid of an adapter.

jrturner514 wrote:

How much faster is the spare HDD over the one currently in the Mac?

I don't know. It might not be any faster. On top of that, it won't even get the tiny little bit of assistance that the internal Fusion Drive is getting from its 24 GB SSD.

I saw a lot of post about people using SDD for external drive application. Is the HDD the weakest link in my plan...just because it was free doesn't mean its the best use for it.

It is definitely a weak link in the plan, if you plan to use it as an external startup drive. When a computer does lots of jumping around from one place to another on the drive, a mechanical HDD has to drag read/write heads around using a motor, and wait for platters to spin data into place underneath the heads. This happens much faster than on a floppy disk – but it still takes time.

A SSD stores data by electronic means, and its random-access times are much, much lower than those of a HDD. So a SSD will slaughter a HDD for applications like starting up a computer (from power off) or launching programs. For data storage, the practical advantage is often not as great – and there, people might go with HDDs because of factors like larger capacity, lower cost per GB, or better ability to retain data for long periods (years) with no power.

You can get a 1 TB external SSD for about $100 USD, plus or minus. Maybe less if you're talking about a USB 3.0 / SATA one.

Do you recommend any good step by step tutorials/resources to guide me through what I'm trying to accomplish with what I have on hand? Specifically order of operation, ie ram before, switching to HDD for macOS. I'm good at following directions but wouldn't want to go about this with out all the sequential steps...particularly concerned with lost data or somehow locking myself out of access.

You don't need to erase your internal drive at all. Just get an external drive, "clone" your current startup drive to it, select the external drive as your startup drive, and reboot. Eject the internal drive (if you like) after rebooting - so that you don't accidentally use it.

Adding more RAM will help, though not if the hard drive is failing as indicated as a possibility in your other thread. If it is failing, then installing macOS on the exernal hard drive and running from there, along with the RAM upgrade, will give your iMac a bit more life. The added speed of the Seagate drives may be insuffient to compensate for the perfomance of the Fusion drive currently in your Mac so you may not see performance increase there. Any working drive will be better than a failing one, of course.

At nine years since your iMac was first released, whether it make sense to spend money on it is something you need to consider. I have the same model of iMac that you do and while it's been serving me well, I wouldn't spend any money on it after all these years. It's certainly not worth the hassle and expense to try and upgrade the processor, if that is even possible.

Regards.

The Exos hard drive is one of the faster hard drives out there so it may be sufficient depending on your needs (I forget how fast it was when I tested one --not under real world conditions -- maybe 200MB/s?), but it still falls short of even a good SATA III SSD (there are a lot of junk SSDs out there even from respected name brands).

Memory usually won't help much for performance. Like @varjak paw points out, you can easily determine if memory is the bottleneck by monitoring memory in the Activity Monitor app after using the computer on your most memory intensive workloads and even after a week of the computer running 24/7.

Macs are also very picky about the memory they use, so most higher level forum contributors will only recommend using memory from Crucial or OWC (the memory must be specified as being Mac compatible for a specific Mac....never buy memory based only on technical specs since the commonly published information is usually insufficient for true compatibility).

More than likely you have a failing internal hard drive. Again, that is very easy to confirm by using DriveDx (free trial period) to check its health. Any "Warning" or "Failing" conditions mean the hard drive is worn out or failing respectively....either condition means the hard drive should be replaced. Unfortunately determining the health of an SSD is not quite so easy.....health monitoring apps can only alert you to manually check & interpret the health information since not all "warning" or "failing" conditions are bad for an SSD.

FYI, you are missing the Hard Drive report, but I saw it on your other linked thread. Both drives' health reports look Ok, nothing stands out in them. However, on rare occasions a hard drive may have issues even when the health attributes look good. Part of that reason is due to some of the health attributes having behavior we don't know how to interpret until they actually trigger a SMART failure status on the drive.

I would suggest running First Aid on the Fusion Drive item, then on the hidden APFS Container within the Fusion Drive. Even if the First Aid summary says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If so, run First Aid again until the errors are gone. If after several attempts the errors remain, then you will need to run First Aid while booted into Recovery Mode. If the errors remain after several attempts, then you will need to start over with a clean install which means erasing the Fusion Drive followed by reinstalling macOS & restoring from a backup. Personally I would go one step further and erase both the physical SSD & Hard Drive, then recreate the Fusion Drive followed by reinstalling macOS & restoring from backup.

As I mentioned, your external hard drive may well not be any faster than the Fusion drive in your Mac, which combines an SSD with a "spinning" hard drive to get better performance at what was at the time a significantly lower cost than having all-solid-state storage. Without objective measurement I can't say with certainty, but if your internal Fusion drive was working perfectly, I'd be a bit surprised if you saw any significant irmprovement going to that external. Of course, as I said, anything would be faster than a afiling drive.

I'd suggest first you continue to work in the other thread to determine where the performance problems do indeed lie. Then we can point you to the next step in the process.

Regards.