Imac Late 2014 NVMe upgrade

You can take out the HDD. That's how the SSD-only configurations were shipped (no HDD). Alternatively you could leave the HDD in and use the two as separate logical drives (not fused).

I'm getting 1150MB/s read and 1000MB/s write with a 500GB Crucial P2 in a 2015 Macbook Air using a JESOT M.2 adapter. Its not the fastest drive and is running a bit slower than hoped/advertised, but it is still faster and higher capacity than the 128GB OEM drive it replaced so I'm happy. Also for a laptop I wanted something with above average power efficiency.

Of course, accessing the SSD in a 2015 MBA is a LOT easier than in your 2014 5K... are you sure using the external SSDs for work isn't fast enough?

Make sure you have a data backup (and possibly also a USB installer) for recovery after replacing the internal drive.

I would strongly recommend against installing an internal SSD. It is much easier and safer to use an external SSD instead. Please read: How to Setup and Use an External SSD as y… - Apple Community. If the LaCie SSD in the capacity you want is out-of-stock, look at this OWC external SSD: OWC Envoy Pro Elektron 1.0TB - MacSales.com (OWC).

-Jack

Here is the install video page for your model from a trusted vendor (and the two parts that are possible to do):

https://eshop.macsales.com/installvideos/imac_27in5k_14late/

Professional install recommended.

You would have to nearly disassemble the entire iMac to get at the NVME adapter. Not worth any costly mistakes that are lying in wait for you. Why not go for a fast external USB3 drive, as you have Thunderbolt 2 ports and no one is making TB2 drives now.

Choose an SSD from OWC (MacSales) or Crucial, and use Carbon Copy Cloner 6 to clone your internal drive to the external drive, set the startup drive to the external, and march on.