[mkgmap-dev] Problem with --index and 812KB _mdr.img files & "warning using default sort"

1. Could mkgmap clean up broken 812KB mdr_img files if creating crashes due to insufficient memory? This is a potential threat as the _mdr.img is broken but hard to notice if you don't look at error notices). 2. What does the "warning using default sort" thingy mean. Can it be overcome or if not could it be switched off?