A system is provided for reading and writing sectors which may be realized as either a disk device to the local operating system, or as a virtual disk device to a virtual machine. A user's computing environment is stored in the network in the form of a disk image, which may be a virtual disk image, for example. The virtual disk is realized on host computers through host-resident virtual machine monitors such as MICROSOFT VIRTUAL PC.RTM.. Portable memory devices, such as flash devices, buffer virtual disk writes and cache reads, greatly reducing the performance degradation associated with remote disk access. The cache is mobile so that it can be travel with the user. The flash device remembers commonly used virtual disk content fingerprints so that the host machine's local disk can be used to satisfy many common disk reads when ubiquitous static content is involved. Standard, frequently used software images might be distributed in advance to host machines. Flash disk contents are trickled back into the networked parent disk image in order to provide an automatic backup facility, adding the benefit of ensuring that the flash drive doesn't overflow.