If I use the --index option with 1268 on any "big" country file from Geofabrik, the mdr file is not created:
"D:\Garmin\mkgmap_680>start /low /b /wait java -enableassertions -jar -Xmx2800M mkgmap.jar --max-jobs=3 --latin1 --index --ignore-maxspeeds --ignore-turn-restrictions --remove-short-arcs=1 --road-name-pois=0x2f2d --locatio
n-autofill=1 --description=openmtbmap_alp_09.10.2009 --route --country-abbr=alp --style-file=new2 --country-name=alps --mapname=65280000 --family-id=6528 --product-id=1 --series-name=openmtbmap_alp_09.10.2009 --family-nam
e=mtbmap_alp_09.10.2009  --tdbfile --overview-mapname=mapset00 65280*.osm.gz
There is not enough room in a single garmin map for all the input data
   The .osm file should be split into smaller pieces first."

Splitting offers of course no help. The above is against alps.osm.bz2 from Geofabrik. (same problem with UK,NL,DE)