Writeback to a virtual media mapped image in HTML5

A system and method for transferring host data from a host server 16 to a user device 12 during a KVM session, using an HTML5 KVM/virtual media client 20 running in a web browser 14 of a user device, the web browser has a temporary sandbox 26 and a persistent sandbox 22; original data from the client is modified with host server data to produce modified bytes which are stored as a disk image on a virtual disk; the client then sends instructions to the host server for it to package and send the modified bytes back to the persistent sandbox of the web browser of the client, initiates unmapping of the virtual disk (334, fig.3A), selects a client storage device to receive the packaged bytes from the persistent sandbox and then sends those packaged bytes to that storage device. The system bypasses the inability of the read-only virtual drives to transfer files, such as log files and other data files, back to a HTML client of the user device. Preferably, disk images are in FAT32 format and the original data is a USB drive image uploaded in blocks on-demand using a reverse lookup. Client-server communications may use Avocent RTM Virtual Message Protocol (AVMP).