Is it possible to write to another partition in windows?
You mean to write to a non-FAT partition, like an NTFS, HPFS, or POSIX partition? You'd probably have to write a VxD to link into the Real Mode interrupt 13h, function 42h (extended read) and 43h (extended write). If there's a prewritten VxD for this (which there probably is), you will have to figure out how it works, but without a VxD, I don't think it's even remotely possible. :)
You can do just absolute reads and writes to a any partition in windows by using the a special VxD for this purpose from ring3. (it is something like VWin32.VxD - last time i looked on microsofts site about disk IO in Ring3 it was about all the information i could find so you shouldn't have any probs finding info about it.) Prewritten OS support for these different file system formats (what i mean is that, there wont be a VxD there already to process advanced commands such as ReadFile etc) should be about nil, so you will have to write your own.