When you hot-add memory to a server that supports this functionality, Device Manager shows the memory as installed, but it is NOT available. The System event log contains a warning similar to the following, but the memory range may be different:

Event ID: 0
Event Type: Warning
Event Source: PNPMEM
Description: Failed to add memory range starting at 0x40000000 for 0x40000000 bytes. This operation exceeded the maximum memory capacity of the operating system or the server is not configured to support dynamic memory operations.

The behavior will occur if the Boot.ini entry that started the server contains the /3GB Switch.

The only way to make the memory available is to restart your server.