What does disk IO do to a string and how can it be replicated?

This is NOT intended to be a "bump." I cannot edit my original post, and after being away from the problem for a few hours, I reliazed my first post was confusing to anyone who wasn't in my mind. This is more simplified. I am new here, and apologize in advance if this is against protocol. I would edit my original post if I had the ability to. Thanks for understanding.

Thanks for taking the time to read this. I am stumped and at wit's end! I'm developing a limited NNTP client to download multi-part y-Enc binary data. Obviously, the parts have to be decoded and processed and hashed and crc'd. With so many IO streams, I'm trying to find a way of keeping the segments in memory until time for joining. That's where the trouble is.

I have the data for each segment stored in a generic list of strings (in 3.5).

List<string> segData;

Now, if I save the string to disk and then load them in the same order, the process WORKS FINE:

StreamWriter sw = new StreamWriter(FilePath, false, Encoding.GetEncoding("iso-8859-1")); sw.Write(segData[i]); sw.Close();
input = new StreamReader(FilePath, Encoding.GetEncoding("iso-8859-1"));

Once again, the above code works fine, but the whole point of this thing is to AVOID saving the individual segments to the disk.

I've tried everything I can think of to avoid the process:

byte[] byteArray = Encoding.GetEncoding("iso-8859-1").GetBytes(segData[i]);
MemoryStream ms = new MemoryStream(byteArray);

SHOULD work in my mind, but it doesn't -- I get CRC errors in the joining and processing.
If you can think of what the write/read cycle is doing to the data to make it work, I would really appreciate it, because this is just WAY frustrating to me.

Thanks in advance for any help or information you can provide.