why would ntwritefile return insufficient resources.

1>From usermode

2>my open call:
status = NtOpenFile(&filehandle,
SYNCHRONIZE | FILE_READ_DATA | FILE_WRITE_DATA,
&objattrs, &iostatus,
FILE_SHARE_READ | FILE_SHARE_WRITE,
FILE_SYNCHRONOUS_IO_ALERT);

3>My write call:
NtWriteFile(fd, (HANDLE)NULL,
(PIO_APC_ROUTINE)NULL, (PVOID)NULL,
&iostatus, buf, (ULONG)nbytes,
&nt_offset, (ULONG *)NULL);

4>the wraper function handles the sector alignment.

5> My NtWriteFile fails "0xc000009a" : “insufficient resources"
I am trying to write (approx 85K) sectors in 1 go.

-any comments appreciated.

just fyi..
The NtWriteFile may fail if the write was on a volume, not a file.
i.e. I had initiated direct write to volume to defrag it. hence the error.