Excuse me for bumping this thread.

Trying to upload ORC based polar file and get an "error with polar conversion".

Should I reformat this file from a .txt?  

You don't need cdo merge - you can just cat the files.

Ended up using CDO utility to convert the netCDF files to grib1. I did this on the Mac as the CDO install is straight forward and this is a shell script to process a series of separate netCDF files for the years/dates of interest. As these are grib files you can then merge when into a single file for easier handling in Expedition

#
for f in cmems_2*.nc
do
  fname=`basename -s .nc $f`
  echo $f $fname
  # set missing paramaters to 0
  cdo setmisstoc,0 $f mcds-input_ex.nc

  # Convert each variable with proper parameter ID
  cdo -f grb1 -setparam,49 -selvar,uo mcds-input_ex.nc u10.grb1
  cdo -f grb1 -setparam,50 -selvar,vo mcds-input_ex.nc v10.grb1  
  
  # Merge into one file
  if [ -f ${fname}.grb1 ]; then
     mv ${fname}.grb1 ,${fname}.grb1
  fi
  cdo merge u10.grb1 v10.grb1 ${fname}.grb1
done

cat cmems_2*.grb1 > cmems_fastnet.grb1

btw I've used cdo merge as this ensures that showvar lists both var49 and var50. If you cat the files this will only list the first variable. You can also use -O on the merge to force existing files to be over written

cdo showvar cmems_2023.grb1
var49 var50
cdo  showname: Processed 2 variables [0.00s 12MB]