Hi @jfw2 ,

Check out page #29 of the User's Guide:

Maximum memory 64 GB with optional memory riser cards

There's that, but see this for comparison:

https://www.dell.com/downloads/global/products/precn/en/precn_t7400_specsheet.pdf 

Several other Dell sources online, as well as nearly all memory vendors suggest that 128GB should be possible.

Hi @jfw2 ,

You are right. I further search on Userbenchmark: Precision WorkStation T7400 builds and do located several T7400 run with 128GB RAM, like this and this. 

Several ways for further diagnostic on your case:

• Refer to this thread, similar problem on a T7400 solved by resitting memory module. Note that this may only works on cases with non-identical RAM, i.e. mix RAM from different manufacturers. Therefore may not apply to yours.
• Refer to another thread, user fix it by BIOS reset by PSWD jumper on motherboard.

See if information above provide any help.

Hi again,

tried those. Even went so far as to remove the cmos battery.

For the ram, I removed all the sticks scrambled the order and reseated them (They're all the same P/N)

No difference still 64 GB available, 64 GB hardware reserved. I notice that the two examples on the benchmark site use X5492 processors. I wonder if that's a clue. Shouldn't be, but...

Just for the historical record, I isolated the problem to a bad (apparently a poor reproduction) secondary riser power cable. Everything works now.