Read-only Definition A read-only file or storage device contains data that cannot be modified or deleted. While data can be accessed or "read" from a read-only file or device, new data cannot be added or "written" to the device. Most operating systems, such as Windows and OS X allow you to mark individual files as read-only. In Windows, you can right-click a file and select Properties to view more information about the file. You can click the Read-only checkbox (located in the General ta...