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I just upgraded the CPU of my 2015 27" Retina iMac and while I was in there I took out the 1TB HDD and replaced it with a 512GB Samsung 850 Pro SSD. Now I'm ready to set it all up, but I'm not sure how to best proceed.

  • I know the SSD will be on a relatively slow SATA III bus
  • I know the blade SSD is 24GB and on a faster bus
  • I know I could use external Thunderbolt for a faster experience (should have considered this but too late)
  • I've read mixed opinion on merging the two SSDs into a fusion drive, but most people suggest against it, since:
    • the speed of both drives will be reduced to the slower of the two
    • if one drive fails I will lose everything

I currently think I should keep them separate and use the 24GB SSD for the OS and use the other SSD for the home folder. Does this seem like a good way to proceed?

I am not sure which file system to use (APFS?)

I am not sure if I should be doing something with TRIM to extend the lifespan of the drives

If anyone can help guide me I would appreciate it. Thanks.

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    1) Yes SATA III is a bottleneck for PCIe SSDs but you won't notice it in normal use. Your R/W speeds will be very impressive. 2) Use APFS. That's the current FS protocol Apple has transitioned to. 3) 24GB is tight for the OS but it will work so long as it is bare bones OS and you're not installing other Apps on it. All other non-OS apps need to be installed on the Samsung drive. 4) Don't merge them into a fusion drive. Use the 24GB as your boot volume. 5)Do not do s*** with TRIM, not needed for what you're running (you aren't making a hackintosh, eg). – njboot Sep 2 '18 at 4:15
  • I formatted them both APFS and went ahead with the Samsung as the main drive (before that I tried using the 24GB as the main drive, but after the OS was installed there was only 8GB or so left, which somehow quickly went down to 5, not enough to install High Sierra, so I aborted that plan). Now I have a bit over 20gb available on the Apple blade SSD. Maybe that would be a good place to move the applications to, if that's something I can do. I have not found a good answer on whether or not it's ok to move the Applications folder. – sshanky Sep 3 '18 at 6:42
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Learnings:

  • There doesn't seem to be any benefit to merging two SSDs, other than having one contiguous space.
  • The disadvantages of merging are that 1) both drives will have the slower of the two bus speeds, and 2) if one drive fails everything is lost
  • The small 24GB SSD that Apple includes in the 1TB Fusion Drive setup is too small to act as the startup drive. After installing Sierra, there was about 8GB remaining, and that quickly dropped down to around 5 once the update to High Sierra downloaded, leaving insufficient space to actually install High Sierra. Maybe I'll use that space for some applications.
  • It doesn't seem that the relatively slow 6GB/s speed of SATA III will have any noticeable impact
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    About this idea to merge two SSDs as a single Fusion drive: I don't get the claim "speed of both drives will be reduced to the slower of the two" that we can read everywhere on this subject. In a traditional Fusion drive (small & fast SSD + big & slow HDD) the speed of the fast SSD is not "reduced to the speed of the slower HDD". Not at all! It is quite the opposite, everything feels snappier as most important and most used files are always in the fast portion. Please clarify this point. – flux_capacitor Nov 17 '18 at 0:13
  • My understanding is that, for any fusion drive, the bus speed of the entire drive is equal to the slower of the two bus speeds that are used to make it. It appears that late-2015 iMac has has a Serial ATA (6 Gb/s) connector for a 3.5" hard drive and a PCIe connector (PCIe 2.0 x4 NVMexpress interface) for the SSD. Of course even 6 GB/s is much faster than the "big & slow HDD", and the fusion drive with the SSD can take full advantage of this, where the HDD alone cannot. – sshanky Nov 17 '18 at 4:07
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    Thank you for this answer. Unless you have a custom SSD controller, you are correct that concatenation game or applying RAID to consumer drives has far more drawbacks than benefits. – bmike Jan 5 at 21:12

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