A single virtual storage device file system that abstracts multiple RAID groups of physical storage devices into one virtual device and one first blockset having a plurality of data blocks in a contiguous linear address space is converted into a multiple virtual device file system that abstracts the multiple RAID groups of physical storage devices as separate multiple virtual storage devices each having a separate second blockset and address space, by migrating data in allocated blocks at boundaries of the physical storage device groups to free blocks, partitioning the first blockset at the boundaries into the multiple second blocksets, updating the block metadata of each block, and rebuilding the file system using the block metadata to generate second blockset metadata.