Abstract: An I/O device is shared between a host operating system, and a foreign operating system that runs under the host operating system, in a single computer. The I/O device must be of a type which includes a port driver that receives device commands for reading addressable fields on a removable data storage media. Each media which is to be a source of data for the host operating system stores a first identifier in a particular addressable field, and each media which is to be a source of data for the foreign operating system stores a second identifier in a particular addressable field. A registry in the host operating system indicates that the I/O device is to be used exclusively by the host operating system; so the host will try to use the I/O device. But in addition, a different private registry in the foreign operating system indicates how that operating system can address the I/O device; so the foreign operating systems will also try to use the I/O device.