OWC 128GB (4 x 32GB) PC4-21300 DDR4 2666MHz SODIMMs ram memory upgrade

• Data Rate: 2666MT/s
• I/O Clock Speed: 1333MHz
• Registration: Non-registered
• Error Correction: Non-ECC
• Cycle Time: 0.75 ns
• Form Factor: SO-DIMM
• Type of Memory: DDR4
• PC Speed: 21300
• Timing: CAS = 18.0
• Full Parameters: 18-18-18-14
• Bandwidth: 21.3GB/s
• JEDEC: Fully compliant with JEDEC specifications
• Capacity: 128GB (32GB x 4)
• Voltage: 1.2 Volts
• Pin Count: 260-pin
• Printed Circuit Board Type: Low noise 8-layer PCB
• RoHS Compliant: Yes
• Ambient Temperature: Operating: 32°F to 185°F (0°C to 85°C)
• Note: PC4-21300 DDR4 2666MHz SO-DIMMs will operate at 2400MHz speeds in iMac19,2 models with 3.6GHz Quad-Core i3 Processor. This memory will operate at full 2666MHz speeds in other iMac19,2 models.

• iMac:
  • iMac (Retina 5K, 27-inch, Mid 2020) Model ID: iMac20,2 - A2115 - EMC - 3442
  • iMac (Retina 5K, 27-inch, Mid 2020) Model ID: iMac20,1 - A2115 - EMC - 3442
  • iMac (Retina 5K, 27-inch, Early 2019) Model ID: iMac19,1 - A2115 - EMC - 3194

• PC Compatible
• Notes:
  • For 27-inch iMac (2020) models – iMac20,1 and iMac 20,2 – OWC recommends that all DIMMs be the same capacity, speed, and manufacturer to ensure optimal memory performance.

🧠 Installing RAM in a 2019 iMac (Up to 128GB)

Step-by-step guide to ensure a smooth upgrade

Power off your iMac and unplug the power cable.

Disconnect all peripherals — USB, Thunderbolt, drives, monitors, etc.

Press and hold the power button for 10 seconds (to discharge residual power).

Install just one RAM module in an empty slot.

Plug in the iMac and power it on. Let it boot fully into macOS.

Shut down the iMac again.

Add a second RAM module, then repeat the power-on and full boot process.

Continue this one module at a time until all RAM is installed.

Once all modules are installed and your iMac boots successfully, reconnect your peripherals.

💡 Why this method?
Installing all 128GB at once can sometimes prevent the iMac from booting. This step-by-step method helps your system initialize memory properly and avoids false boot issues.