When you're working on a file, it's saving constantly to RAM but also hopefully to disc, the hard disk so you don't actually lose it if the power goes off.

当你打开一个文件，它不断地保存在内存中,但对于光盘，硬盘,当你断开电源的时候，数据不会丢失。

哈佛公开课 - 计算机科学课程节选

The 4420 will also offer "random erase technology" that lets you use a CD-RW disc just like a floppy or hard disk -- adding or deleting individual files at any time.

CNN: Verbatim to ship first 4X CD-RW drive

"Over time, an optical disc will be as much of an historical curiosity as a floppy disk, " said Michael Gartenberg, a tech-industry analyst with research firm Gartner Inc.

CNN: Why disc drives are an endangered species