A resposta à sua pergunta está no artigo da Wikipédia sobre os dispositivos periféricos aos quais você está vinculado:
A peripheral is a device connected to a host computer, but not part of it. It expands the host's capabilities, but does not form part of the core computer architecture. It is often, but not always, partially or completely dependent on the host.
A peripheral is generally defined as any auxiliary device such as a computer mouse, keyboard, hard drive, etc. that connects to and works with the computer in some way. Other examples of peripherals are Expansion cards, graphics cards, computer printers, image scanners, tape drives, microphones, loudspeakers, webcams, and digital cameras. RAM - random access memory - straddles the line between peripheral and primary component; it is technically a storage peripheral, but is required for every major function of a modern computer and removing the RAM will effectively disable any modern machine.
Como você pode ver, interno e externo não se referem ao gabinete do computador, mas à arquitetura central do computador. Como regra geral, qualquer coisa que possa ser removida sem danificar o computador de impedi-lo de inicializar não faz parte de sua arquitetura central e, portanto, periférica.
O "oposto" de um barramento de expansão seria um barramento interno, como aqueles entre CPU e Northbridge, Northbridge e Southbridge, CPU e chipset (todos presentes) ou o barramento de memória.