It seems there is a workaround for this. One can buy an external Thunderbolt SSD, and set up a Fusion Drive. Since Thunderbolt is extremely fast, this is a low cost, low work solution that is very suitable for desktop machines like the iMac.
There are a lot of guides online on how to set up the disks.
It boils down to
- Backing up your machine using Time Machine
- Install OS X onto a third external drive.
- Boot from said drive.
- Create fusion drive over external SSD and internal HD.
- Reboot from fusion drive and re-install OS X, restore from Time Machine
Maybe one can skip the install to external HD, since newer Macs come with Internet Recovery. This would save quite a bit of work.
Once in recovery, you can see your disks using diskutil list. Let's say you see the disks disk1s2 (internal HD, OS X partition) and disk2 (clean SDD).
Now you can create the Fusion Drive:
sudo diskutil cs create [ArbitraryName] disk2 disk1s2
sudo diskutil cs createVolume [UUID] jhfs+ [ArbitraryName] 100%
The output of the first command results in a UUID which you use in the second command.
After this, you should be able re-install or restore OS X onto the Fusion Drive.
I haven't tried this yet, it is merely a summary of the linked article! So be careful and make backups!
Because the maximum throughput of any port on the iMac 5k Late 2015 is only 10Gbps (Thunderbolt 2), no external storage solution can match the internal SSD's speeds.
The short answer is that you would give up a lot of performance.
The internal SSD can transfer data at about a couple GB/s.
An external Samsung 850 Pro 512GB via a self-powered USB 3.0 docking station runs at about 400MB/s during CrystalDiskMark sequential transfer benchmark on an exFAT partition.
That said, for running macOS and applications, the difference between the internal SSD and an external SSD is much less noticeable than that between an SSD and a traditional HDD. Unless you are constantly moving large files on and off your drive, an external SSD won't feel much slower subjectively than the internal SSD.
You did not specify how much data you have or your budget, so I cannot give specific recommendations. But in general, I put macOS and applications on an internal SSD. I would figure out how many GBs macOS and all my applications would consume, and choose an SSD of appropriate size.
If you really value the drive being internal and need large capacity on a budget, consider the 2TB or 3TB Fusion Drive. They have a 128GB SSD built in and intelligently find and put the data you access the most frequently on the SSD portion. The 1TB Fusion Drive only has a 24GB SSD so is less attractive
Thunderbolt 2 enclosures doesn't make much sense
True, an SATA SSD could be bottlenecked by USB 3.0. But not significantly so. That is because SATA itself tops out at 6 Gbps (SATA3), not much more than USB 3.0's 5 Gbps.
For the same reason, even though Thunderbolt 2 eliminates the link bottleneck, the benefit would be small because now the bottleneck is on the SSD side, which is 6 Gbps.
There are PCIe NVMe SSDs (the internal SSD falls into this category), which are faster than SATA SSDs. But to use those externally you still have two problems:
- Thunderbolt 2 tops out at 10Gbps, slower than what the internal SSD and other PCIe NVMe SSDs are capable of.
- There are few (none?) Thunderbolt 2 to PCIe NVMe SSD enclosures. The StarTech product you linked to takes SATA drives.
All Thunderbolt 2 enclosures I have come across are ridiculously expensive, including the one you linked to, especially considering the limited performance gains. You would be much better off spending the money on upgrading the internal SSD.
Best Answer
Only APFS is supported for running Catalina. There may be some weird workaround for running Catalina on HFS+, but I don't know of any.