Before showing you the last command, let me warn you that CD-writers want to be fed with a constant stream of data. So the process of writing the CD image to the CD must not be interrupted or a corrupt CD will result. It's easy to interrupt the data stream by deleting a very large file. Example: if you delete an old CD-image of 650 Mbytes size, the kernel must update information about 650,000 blocks on the hard disk (assuming you have a block size of 1 Kbyte for your filesystem). That takes some time and is very likely to slow down disk activity long enough for the data stream to pause for a few seconds. However, reading mail, browsing the web, or even compiling a kernel generally will not affect the writing process on modern machines.