sync

File systems don't write your data to the disk directly. They buffer it until later and then write it. Thus, block level snapshots of your disk can be incomplete.

Sync can be used to insure file system integrity, but it buffers a message to write to disk instead of waiting to return until everything is written to disk. The fsync system call takes a file descriptor as an argument and won't return until data is written to disk. Chapters 3.14 and 4.24 of APUE cover this in more detail. EMC seems to advise (page 10) that you sync and umount a disk before using their block-level SANcopy tool.