in basics when writing to rewritable CD's.....
You can either use a "packet writing" programme, such as "Direct CD", (or the equivalent from Ahead/Nero, called InCD), or use a more "conventional writing prog, such as Easy CD Creator, or Nero...
If you arre using a "packet writing" prog then the CD has to be formatted first. (The formatting is known as UDF, Universal disc format.) When this is done then the formatted CD is written to in exactly the same way as you would write to a floppy, eg "drag and drop". The formatting process has other consequencies;
1) To read such a CD the PC must have software that can read UDF discs. This will be achieved by having either "Direct CD" or "InCD" installed, or a generic UDF reader. (and probably others of which I am unaware)
2) Formatting takes some space, so a formatted 650M disc only has about 500M useable space.
3) A disc so formatted cannot be used with "conventional" CD writing software, (Nero, or Easy CD Creator), as you have discovered without first being completely erased.
I hope this helps. I add 2 riders...
I know that Windows XP has it's own CD writing capabilities that is different to both the above but I don't know how it behaves.
I know that some "packet writing" software will also format CDR's (ie, not rewritable discs). Again I'm not sure of details of this.
I hope this helps!