Data disks and/or CDs are sometimes made unreadable

I am using Windows XP Professional. The problem is -
that after using a correctly formatted disc in (a: drive)
or a CD (e: drive) and saving files to it, up to a
certain point it works fine. Then when I use the disc or
CD another time, and open the disc or CD to save more
files to it, I get the message: "Disk in Drive A: is not
formatted - or has been formatted for a Macintosh" I hit
OK and the second message I get is "You do not have
access to the folder "A:\". See your administrator for
access to this folder" I think that the message
should be that the disc is full - but Windows XP somehow
makes the disc or CD unreadable in Windows XP. I can go
to another computer with Windows ME and the disc or CD
opens OK.