Assumig DIR holds input directory and MOUNTED holds a mount point
${DIR#$MOUNTED} should return:
a.- a relative path from MOUNTED to DIR, if it exists (DIR below MOUNTED)
b.- DIR
Of course, / will always return a relative path. The script should go through all mounted drives and take one of them, or else take /.
I'm not sure if this will work inside awk, but it works fine in the shell script.