Transfer SSD from one 2014 iMac to another 2019 iMac ( swap )

·       Always connect your old Mac's Time Machine External Hard Disk prior to startup, this will allow a migration from your TM backup and it will be dead simple! When you start the new Mac for the first time it will startup Setup Assistant (SA), SA will get to a point where it asks if you are migrating from another computer, select Yes and follow the on-screen prompts and it's done! It is that simple! This is the easiest, quickest and most reliable method to migrate from the old Mac to the new Mac is to simply connect the old Mac's Time Machine External Hard Disk to the new Mac using a USB cable before you turn on the new Mac. 

·       NOTE 1: You will likely need a USB A to USB C adapter, you can find them on Amazon for $10 or less.

·       NOTE 2: If you have already done SA on the new computer then simply connect the Time Machine External Hard Disk using a USB cable and launch Migration Assistant (MA) which is located in Applications - Utilities - MA. Then follow the on-screen prompts.

·       The ONLY difference between MA and SA is MA will create a new user account that you need to log into.

Both SA and MA migrate applications (not 32 bit apps), data files & settings. They do not migrate Mac OS itself. Some professional apps such as Adobe and Microsoft apps may require re-installing their apps as they will not allow them to be migrated. As long as you kept the security keys for installing their apps, it is not an issue. 

Finally, never EVER use Wi-Fi when doing Setup Assistant or Migration Assistant. Why, it simply fails in most cases. Please read and follow the directions in Setup Assistant and Migration Assistant. If you do not have a Time Machine backup of the older Mac then create one! If you are not aware of Time Machine, please read Back up your Mac with Time Machine and finally if you need an External Hard Disk to use as a Time Machine External Hard Disk then many experience users on these forums prefer the OWC Mercury Elite Pro due to it's reasonable price, high level of quality and ease of use. 

If for some reason you never backed up the old computer using Time Machine or a bootable clone you could remove the old Macs SSD, install it in an enclosure such as the following (select the one with no SSD inside) and migrate using that. https://eshop.macsales.com/shop/owc-mercury-elite-pro-mini/usb-c

If you never backed up the old machine, then begin with the new one the day you get it!!!!!!!!!!!!!!!!!

MacTracker says that

• The iMac (Retina 5K, 27-inch, Late 2014) has "1 - 6.0 Gbps Serial ATA (SATA), 1 - 5.0 GT/s PCIe x2"
• The iMac (Retina 5K, 27-inch, Late 2019) has "1 - 6.0 Gbps Serial ATA (SATA), 1 - 8.0 GT/s NVMe PCIe x4"

You could transfer a 2.5" or 3.5" SATA drive (HDD or SSD) and hope that the new Mac would boot from it. (A 2019 iMac requires at least macOS 10.14.4 (Mojave).). A safer course of action might be to put the drive taken out of the old machine into an external enclosure, and to install a new drive (one not as old) into the new machine.

If you are talking about a blade SSD, I'm not sure whether you could transfer the old SSD or not.

Note that opening and resealing either iMac is a job involving major surgery and is best left to a repair shop.

Keep in mind that one of the most common reasons for old computers to fail is drive failure. Especially mechanical hard drive failure, but SSDs can go bad as well. If you do not have backups, you can try getting data back from the old drive, but it's a gamble.

With recent and current Macs, there is basically no chance of "pulling the drive" to recover data. The data stored on the internal SSD is encrypted with a key held by the Apple Silicon processor or T2 security chip. Separate that data from that key, and it is so much unintelligible hash. Furthermore, on most of these Macs, the flash chips that hold the encrypted data are soldered to the motherboard. This underscores the importance of doing backups, so that if something happens to the Mac or its internal drive, you have a way to recover your data.

Follow these guidelines:

Transfer to a new Mac with Migration Assistant - Apple Support