With Cubase it pays to be very careful where files are created. If your focus is on audio (as opposed to MIDI and Instrument Tracks) then you really need to understand the Audio Pool (Ctrl-P) … but it’s all in the manual, even the short “Getting Started” PDF.
When transferring files between computers, if you’ve maintained a proper folder structure then it shouldn’t be a problem, but it is still very easy to “drop” files outside of the project folder, which are then referenced in the project but not contained within the folder itself. One way around this is to use the “Backup Project …” function in the File menu – this allows you to specify a completely new folder, into which will be copied all the bits required by the project. You can then safely copy this new folder to another computer safe in the knowledge that it will work.