You state that you are seeing problems when moving the 4250 harddrive into a 4235 appliance.
This is not supported and can lead to unkown consequences.
The physical harddrive can be moved from a 4250 to a 4235, but the image installed on that harddrive has not been made to move from a 4250 to a 4235. So if the harddrive is moved you would need to re-install the base sensor software from scratch (install from CD).
During initial installation the software will detect the type of hardware and configure the sensor for that specific type of hardware. Internal configuration files get modified during installed based on the type of hardware detected.
When installed on a 4250 it detects that 2GB are available on that platform and will optimize itself to run within that memory.
When installed on a 4235 it detects that only 1GB is available on that platform and will optimize itself to run within that more limited memory.
NOTE: The amount of memory is based on the platform type. Trying to add an additional 1GB of memory to a 4235 could confuse the software and is not supported.
Software that has ben optimized for the 4250 will not run properly on a 4235 and will likely generate errors.
(Can't tell you what errors since it not something we support or test.)
If you are going to move the harddrive from one model sensor to another model sensor, then plan on re-imaging from CD so the installation process can detect the hardware and optimize the configuration for that hardware.
NOTE: If your main purpose in moving the harddrive is to receate the configuration on the other sensor, then you might try copying the configuration instead of moving the harddrive.
Copy the configuration of the 4250 to a ftp server, and then shutdown the 4250.
Bring up the 4235 and run setup just to set the initial network parameters and then copy the config on to the 4235 from the ftp server.
Cisco does not sell and does not support additional memory on the 4235s. The IPS 6.0 software was designed to run within the 1GB of memory that is the default on the 4235 sensors. But to run properly it needs to detect that it is a 4235 on initial installation so the software can configure itself correctly for that platform.
Now it is possible that because of tunings you have made in your configuration, that the tunings may have worked in a 5.x version on a 4235 but not on a 6.x version on a 4235.
The new features in IPS 6.0 do take up a bit more memory than was used in IPS 5.1. This means there may be less space memory available in the 6.x sensor for things like Custom Signatures.
If your configuration is one that pushes the sensor to the edge os it's available memory then your options include:
1) Reducing your customizations (removing some of the custom signatures even)
2) Optimizing your custom signatures (many custom signatures are written in ways that consume large amounts of memory and can often be rewritten with the Multi-String engine that consume less than half the amount of memory when done in other engines)
3) Disable some of the new IPS 6.0 features (like disabling Anomaly Detection)
4) Disable signatures that do not apply to your network (filter through the signatures and disable ones that don't apply to the type of machines in your network)
5) Or lastly consider an upgrade to a larger model sensor with more base memory.
The internal settings for a 4250 do not match those of a 4235.
