Why can i put only 115 midifiles on a 256 Mb SD card with the sd managersoftware.
after 115 files i get alarm saying no more space on card......in explorer i see another 240mB left.
Whats wrong????

Many reasons. If there is nothing else on the card see what scandisk says then maybe do a thorough run to find any possible bad sectors and first confirm there is nothing wrong with the card.

i tried several cards, another usb cardwriter from panasonic.......
nothing else is on the card only sd_sound directory with 115 midifiles and 2 other files:sd_sound.plm and sd_sound.tkm
so..................

the files you describe are correct. However if I format a 64 MB card in the 7k to FAT16 then I can put 400 midi files on it taking up around 19 MB of space leaving about 42 MB free for more files. Same on a 128 card only with more free space naturally.