Some of you using Windows XP and OS X in the same environment may have encountered this. In order to have a Mac and PC both able to write to an external USB drive, the disk must be formatted Fat32. Windows XP will not let you format a Fat32 disk larger than 32GB as one partition.
The easiest way to overcome this Windows XP shortcoming is to use OS X to format the drive. If you run Disk Utility, choose your external USB disk on the left and select the Erase tab at the top. There is a Volume Format choice: ‘MS-DOS File System’. Choose this and erase your disk and it will format the whole disk as one partition and be readable and writable by both OS X and Windows XP.
This seems obvious to me now but a few web searches sent me in the wrong direction with third party DOS applications on Windows and unnecessary UNIX commands on the Mac side.