The present application is directed to access isolation for multi-operating system devices. In general, a device may be configured using firmware to accommodate more than one operating system (OS) operating concurrently on the device or to transition from one OS to another. An access isolation module (AIM) in the firmware may determine a device equipment configuration and may partition the equipment for use by multiple operating systems. The AIM may disable OS-based equipment sensing and may allocate at least a portion of the equipment to each OS using customized tables. When transitioning between operating systems, the AIM may help to ensure that information from one OS is not accessible to others. For example, the AIM may detect when a foreground OS is to be replaced by a background OS, and may protect (e.g., lockout or encrypt) the files of the foreground OS prior to the background OS becoming active.