Yes, I was afraid of that. I found an 8 GB stick selling on offerup for $10 and was hoping it would do the job. I've already made the offer, so I guess I'll pick it up and learn the hard way if it will work or not. I do know that sometimes RAM specifications can be surpassed. I hope so this time. 

According to the specs here, no. You can go either 1, 2 , or 4 GB RAM modules to a maximum of 8 GB total.

Oh, thank you very much. My $10 wont go for vain! As far as exchanging the i5 for the i7 ... the two are pretty similar except for the 3.5 GHz frequency for the i7 and 3.2 Ghz for the i5. With the variation within individual CPUs being fairly wide with 2nd generation CPUs, I may be trading in an exceptional i5 for a sick i7 in order to get the computer to run just a little bit faster. I'm not sure upgrading a 10 year old processor is worth much at this time. 

The E6420 is not my main computer. I use it as a "test" machine. In fact, I have 8 computers, with three 8th generation machines, one10th generation, and one 11th generation machine. It often amazes me when the E6420 can actually look good against the younger machines. But still, the memory needs upgrading. 

@rwcoover 

One other option:

Leave one of the 2GB and add the 8GB for 10GB total.

The first 2GB of the 8Gb stick and the single 2GB stick will work as Dual Channel/Dual Interleave.

Just to make sure, I'll test it in the morning.

Regards,

U2

Your very welcome @rwcoover 

To make sure these models will last till 2025 I like to upgrade them to the i7 and 16GB memory.

https://www.cpu-upgrade.com/CPUs/Intel/Core_i5_Mobile/i5-2520M_(PGA).html

In-fact I upgraded 7 E6420 XFR's today. ( 5 regular, 2 Touchscreen )

500+ upgraded 500+ to go!!!

I would like to use SSD drives but I have hundreds of 1TB SSHD's that I use instead.

I'll let you know the test results in the morning.

Regards,

U2

Good morning @rwcoover 

Just tested the E6420.

Model = E6420

Total Memory = 10GB

Interleave = Activated

DIMM A = 2GB

DIMM B = 8GB

Best regards,

U2

U2,

From where can I find an 8GB memory stick to replace one of the 4 GB sticks in my E6420 laptop?

I bought an 8 GB memory stick from Crucial and it did fit into the slot that had the 4 GB stick.

I bought   8 GB DDR4-2666 SODIMM  1.2v  CL-19

Jay

I meant to write "it did not fit into the slot that has the 4 GB memory stick.

Jay

From what I see from your post, you made at least two mistakes. You purchased 
DDR4, which will not fit in your E6420, rather than the correct DDR3, and you probably bought the wrong speed too. You need to go back to Crucial or somebody else and purchase an 8 GB DDR3 PC3-8500 Mhz or, better yet, DDR3 PC3-1066 Mhz