what nav unit are you running Nik? Mark 2, 3 or 4. I assume it is 3.
can you clarify what CD you are putting in and out? Is it the Nav SW or your navigation disk?

Usually the system says remove the SW CD from the drive, then push OK, and the system reboots.
If you push OK before removing your SW update disk from the drive, it will suck it back in as part of the reboot process, and try to load it again.

My understanding of the postcode search feature is that it requires up to date software (V31?) and new(ish) navigation disk.

Hope this helps.
B