We have a T7400 which had had 16GB RAM in it (8 lots of 2GB modules on two 8-slot risers) from when we bought it in 2009. One of the 2GB modules died, so we replaced it with RAM which is identical in terms of spec, although it wasn't bought from the Dell site. From what I've read you're supposed to install the RAM in matched pairs, which we've done (ie, we took one of the working Dell RAM modules out and replaced it with a pair of the new modules); we've ended up with 20GB of RAM, comprised of 12GB of the Dell RAM and 8GB of new modules.
However, the system only shows 8GB as being available, the rest being "hardware-reserved" in Resource Monitor. In other words, it appears to be using only the new RAM and ignoring its original Dell memory. I've taken two of the modules out, leaving 8, in case that was causing problems - and now, in the BIOS, the RAM is listed as being:
| Slot | Size | ECC | Rank | Type | Organization |
|---|---|---|---|---|---|
| Riser 1 DIMM 1 | 2GB | Yes | 2 | FBD | x8 |
| Riser 2 DIMM 1 | 2GB | Yes | 2 | FBD | x8 |
| Riser 3 DIMM 1 | 2GB | Yes | 2 | FBD | x8 |
| Riser 4 DIMM 1 | 2GB | Yes | 2 | FBD | x4 |
| Riser 1 DIMM 2 | 2GB | Yes | 2 | FBD | x8 |
| Riser 2 DIMM 2 | 2GB | Yes | 2 | FBD | x8 |
| Riser 3 DIMM 2 | 2GB | Yes | 2 | FBD | x8 |
| Riser 4 DIMM 2 | 2GB | Yes | 2 | FBD | x4 |
Which seems to indicate the new RAM has a "x4" in the organization column, and all the original RAM is "x8". What does this mean? Can we use the new RAM with the old? What are the potential differences between the original RAM and the new modules, if all the headline details (clock speed, pins, buffered/non-buffered, etc) are the same?