Apparently some large-file systems out there, such as my powerpc64le Linux box, define daddr_t as a 32-bit type, which is sad and stymies cross-building disk images. Use off_t instead, as, ultimately, the value is destined for lseek(). While we could continue to use a 32-bit daddr_t and cast late, that would limit us to 2**32 * BLOCKSIZE byte images.