This might help-- Memory Module Guidelines-- To ensure optimal performance of your computer, observe the following general guidelines when configuring your system memory: • Memory modules of different sizes can be mixed (for example, 2 GB and 4 GB). But, all populated channels must have identical configurations. • Memory modules must be installed beginning with the first socket. From the Dell Manual for your model here-- 

http://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-7010_owner%27s%20manual4_en-us.pdf 

I guess I'll buy another hynix stick to see if it'll work alone with the other hynix. Can anyone tell me if it might solve the issue? Perhaps @speedstep

?

Hey buddy, i actually had this SAME problem and you're 100% right on. i was going to answer what worked for me but you already had it! the cpu was seated wrong and therefor the power was not going to slots 1 and 3. I re-seated the cpu and boom, it booted fine with all 4 slots. To be honest i'm a little upset i didnt see this post before. 

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@SENIORhelpdesk wrote:

Hey buddy, i actually had this SAME problem and you're 100% right on. i was going to answer what worked for me but you already had it! the cpu was seated wrong and therefor the power was not going to slots 1 and 3. I re-seated the cpu and boom, it booted fine with all 4 slots. To be honest i'm a little upset i didnt see this post before. 

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No worries, live and learn lol. I've since switched to a custom rig with Ryzen though, a real beast. 

@Crysium​ I encountered this problem with my Optiplex 7010 where theBlack Pair of Memory DIMM sockets gave a 2-3 Blink diagnostic if populated.

I re-seated the CPU and cleaned and replaced the Heatsink Compound and all is well now.

For anybody about to do this, the procedure requires:

1. Open the Computer
2. Detach the DVD ROM Drive Cables and remove the DVD ROM Drive
3. Detach the Hard Drive Cables and remove the Hard Drive complete with the Cage
4. Remove the 5 screws securing the Systm Fan and lift off the Fan
5. Loosen the 4 Spring Screws securing the CPU Heatsink
6. Clean all traces of Heatsink Compound and clean all dusr from the Heatsink vanes
7. Unclip the CPU ZIF Socket retainer and lift the Pressure Plate; Lift out the CPU and examine for tarnish or dirt on the pads of the CPU and Socket. In my case, both were clean so I simply reseated the CPU in the socket and fixed the Pressure Plate and Retainer Clip 
8. Apply fresh Heatsink Compound and re-fix the Heatsink with the four Spring Screws
9. Re-fix the System Fan five screws
10. Replace the Hard Drive and DVD Drive and Case Cover

Thank you for the tip and I hope this helps anybody els who encounters the same issue.

Unfortunately it did not work for me. 7010 MT.

I first removed and reseated the CPU.

When that didn't work, I removed the CPU and ran over the pads with an emery cloth.

(There was no obvious discoloration or tarnishing of the pads.)

No difference, 2-3 blink continued. Oh well...