issue_comments: 1484583468

First, I totally agree with @jhamman having backend_kwargs on to_netcdf.

For the particular use case, netcdf-c/netCDF4-python create HDF5 files (NECTDF4-format) with track order enabled as required, see https://docs.unidata.ucar.edu/netcdf-c/current/file_format_specifications.html#creation_order.

h5netcdf uses track_order=True as default since version 1.1.0. There have been (and still are, https://github.com/HDFGroup/hdf5/issues/1388) some corner case issues upstream which netcdf-c can somehow circumvent, but h5netcdf can't. Nevertheless, to be compliant with netcdf-c track_order=True is default for h5netcdf.

@abunimeh As a workaround until this is sorted out you could create the file (or subgroup) using h5py/h5netcdf with track_order=False. If a file (root-group) or sub-group in a file is created with track_order=False this will be persistent as it is set at group-define time. Then you can use to_netcdf as usual with mode="a" to append.

```python

import xarray as xr import h5netcdf from time import sleep

ds = xr.Dataset(data_vars=dict(hello=(["x"], [1., 1., 1., 1., 1.])))

track_order = False group = "/track"

with h5netcdf.File("sample1.nc", "a", track_order=track_order) as f1: if group.split("/")[-1]: f1.create_group(group)

ds.to_netcdf("sample1.nc", mode="a", engine="h5netcdf", group=group)
sleep(5)

with h5netcdf.File("sample2.nc", "a", track_order=track_order) as f2: if group.split("/")[-1]: f2.create_group(group)

ds.to_netcdf("sample2.nc", mode="a", engine="h5netcdf", group=group)
```

Update: Use mode="a" everywhere. Update2: Cave: You will not be able to append to this file with netcdf-c/netCDF4-python ever again.