I find that re-writing the field turns my file into a giant 4 GB
	sparse file when re-writing the first field..  doh!

	Any help would be appreciated,

Spend a while making sure your code does what you think it does.

For example, use ktrace to see what arguments you're actually passing to
the system calls in question (lseek and write).  Print out the current file
position (use lseek to find it) and size of the file (use fstat to find
it) before and after each system call.  And so on.

Dave