can I install 2.5" SSD in a late 2013, i7 14,2 iMac in addition to the 500GB NVMe already there

Apple offered a factory blade SSD option that was 512GB in 2013. If yours is 512GB it it likely original. If really 500GB, it may be a replacement of the small blade in a Fusion-equipped iMac.

If it had a Fusion drive the cabling is likely still there. For other hardware see the "kits" here that come with a 3.5" to 2.5" mounting bracket, thermal sensor, and tools:

https://eshop.macsales.com/shop/ssd/owc/imac-27-inch/2013-2019

Also see the links to install videos lower on that page.

If the HDD cable in not there, OWC has one as well:

https://eshop.macsales.com/item/Apple/9230312U/

Many of us who have done conversions like this will ONLY use the OWC Electra SSDs, or Crucial's MX series (NOT the older BX series). Too many other SSDs are not happy in Macs.

If this originally had a Fusion drive, the SSD was only 128GB, so if it now has a 500GB blade SSD, it was replaced at some time. The HDD portion of the Fusion drive was a regular SATA HDD and may be replaced by a SATA HDD or SSD.

Assuming the Fusion drive was removed, there is no guarantee the internal cable or mounting hardware are still inside. You will have to open the case to find out.

sdean7855 wrote:

I think I've pretty much flogged this to the completion of my questions. If your speed eval is correct, then there's little value in the risky business of cracking the case to put in an internal SSD. I've read elsewhere that the Thunderbolt connected SSD is about comparable to a USB 3.0 connected SSD.

The 2.5-inch SSDs are, I believe, SATA3 6GB at best. Under optimum conditions they theoretical can do no more than 600MB/sec. In real-world installs, including results in hundreds of EtreCheck reports here, I've seen no faster than 500MB. It has to be a limitation of the SATA bus compared to NVMe. One is a wheeled locomotive on iron tracks, the other is a MagLev. Different speeds due to different path limitations.

I have no experience with the older Thunderbolt 2 so cannot say what that would do. I know the T3 on 2017 and newer iMacs seems be able to match an internal NVMe.

If your speed eval is correct, then there's little value in the risky business of cracking the case to put in an internal SSD.

I cannot disagree. That would be a sound decision. It would give you the ability to have plenty of routine storage while leaving the NVMe for system files and things that need the speed.

If this is your exact model iMac, you can find instructions on iFixit dot com.

Model A1419 / Late 2013 / 3.2 & 3.4 GHz Core i5 or 3.5 GHz Core i7 Processor, ID iMac14,2

Decent discussion and views of SATA connectors here:

https://en.wikipedia.org/wiki/SATA#SATA_power

Some connectors have both power and data in one housing.

3) How much faster is an internal SSD than an external USB3 connected SSD. Any guess?

No guessing needed. For most installations of an internal SATA 6GB drive, it will do 500MB/sec. That what my 2012 Macbook Pro does with an OWC Electra 6G SSD day in and day out.

External USB 3 reduces that only marginally. You should expect 400MB/sec write/reads from an SATA 6G SSD in a USB3 enclosure.

Your factory 512GB Drive should be doing in the 1500-2000MB/sec range. Maybe more.

sdean7855 wrote:

I'm stunned that a 2.5" SSD is so much slower than the NVMe...I would have thought that, being both SSDs, their speed would be comparable.

The reason is that a 2.5" SSD is connected (and limited) by the 6GB/s SATA connection. The blade SSD is directly connected via NVMe/PCI Express, which is much faster than SATA.

sdean7855 wrote:

This would appear to show that my SM0512f is a little faster than a 1TB Samsung 2.5" SSD. but that the latter is better overall. Comments?

One is not inherently "better" than the other; you would have to compare their detailed specs, cost, user experience, MTBF, etc.

Or is the NVMe SSD connection inherenty faster than a SATA SSD because of how it is tied into the main logic board?

Explained earlier; yes, the blade connection is faster than the SATA connection; but in both cases you will benefit from the full speed of the SSD. Unless you are doing heavy video editing/production or math crunching it would be difficult to notice a meaningful difference in real life.

sdean7855 wrote:

So I'm inclined to believe it came with the NVMe SSD from Apple. If can I connect, a 2.5" SSD and where can I get the cables

As am I. That is a 512GB.