To resolve the issue, add a few megabytes of cushioning to the desired hard quota.
Xsan quotas may behave unexpectedly
Xsan hard quotas may behave unexpectedly with large file transfers.
If you're trying to copy a large amount of data, the process may stop within a few megabytes of your hard quota due to the file system allocation strategy—even if those last few megabytes are available. Subsequent, smaller copies and file creations will be permitted as long as they don't exceed your hard quota.