Late 2015 27 IMac with 2TB Fusion drive. SSD upgrade options?

[DSTOFEL]

I’ve got a late 2015 27 inch 5K iMac (2TB Fusion drive, 24GB RAM, 3.3 GHz i5, Radeon R9 M395 Graphics). This one comes with a 128GB flash drive as part of the fusion drive.

I mainly use this for photos (Lightroom/Photoshop) and videos (Final Cut Pro). The main bottleneck in terms of speed seems to be the HDD portion of the fusion drive. I’m at about TB‘s full on the drive.

Here’s what I’m thinking. Curious if anyone thinks I’m off base with this logic or perhaps has a better idea:

• I think leave the fusion drive intact. My assumption is that the OSX and all applications are contained on the 128GB flash drive portion of the fusion setup which, even on a 2015 iMac, is the fastest option I have for booting OSX and running apps (ie faster than cloning the system onto any external SSD or onto an any internal SSD that I could use to replace the internal HDD).
• Add a 1TB (external) SSD such as “Sandisk Extreme Portable SSD V2”. This SSD is rated at 1050 MB/second. I could connect this via my Thunderbolt 2 port on the iMac but only when used in conjunction with a Thunderbolt 3 dock (ie connect the Sandisk via USB-C to the TB3 “powered” dock and connect the dock to the TB2 port on the iMac using a TB3 to TB2 adapter.
• I’m thinking: Put my Lightroom Catalog, previews and camera raw file on the external SSD and keep my photos on the internal HDD. I could also edit videos on Final cut pro from the external SSD.

My thinking is that OSX and the applications (ie Lightroom, Final Cut Pro, etc..) would run off of the 128 internal flash portion of the fusion drive, my Lightroom catalog, previews, etc... could reside on the external SSD along with any current video file I need to edit with Final Cut. The photos and completed video projects could be stored on the HHD portion of the fusion drive.

This seems like a better solution than simply replacing the internal HDD drive with an SSD (2TB for example) and restoring the system onto this drive (ie to run everything from this drive)....or restoring the system onto an external SSD (ie like a 2TB Samsung extreme portable V2) and running everything from this external device.

Am I missing something in this logic?

 

[DSTOFEL]

Here's what I ended up doing:

• Bought a Sabrent 2TB Rocket NVMe SSD along with a Wavlink Thunderbolt 3 enclousre.
• Bought an OWC 14 port TB3 dock (got an open box new for $190) along with an Apple TB3 to TB2 connector and a TB2 cable.

I cloned my internal drive of the iMac to the sarbent and connected it (inside the Wavlink TB3 enclosure) to the TB3 input port on the OWC Dock. I then connected the OWC Dock to the TB2 port on the late 2015 iMac using a TB3 cable (inserted in the TB3 output port of the OWC dock) connected to the TB3 to TB2 connector...and then a TB2 cable out of the TB3 to TB2 connector and into the TB2 port of the iMac. I'm getting read speeds of around 1270 and write speeds of around 1240 via Black Magic speed test (on big Sur). This transformed the 2015 iMac. For now, I'm going to leave the fusion drive intact inside the iMac. If this solution proves sustainable after some time...I might "un-fuse" the internal drives and use my 2TB internal HD as a backup drive. I guess I could also take advantage of the 128 internal SSD as well.

For anyone that has a more recent mac with TB3 ports: When I connect the Sabrent (enclosed in the Wavlink TB3 enclosure) to my 2018 15 inch MBP (via TB3 port), I'm getting read speeds of: 2280 and write speeds of 2050.