Problem: Mac formats hard disks using HFS+. Windows uses NTFS as the default format. Apparently, you can use FAT or FAT32 formatted disks on both Mac and PC - there are some limitations on files sizes and number of files. But in Windows Vista Microsoft has introduced a new format called exFAT replacing/enhancing the FAT file formats. What a nightmare! I couldn't use my USB external hard drives (NTFS) with the Mac - it could read the contents but could not write anything from the Mac.
Simple solution: If you are having a Windows and a Mac on the same home network, there is a simple way to share files between the two - via FTP. Just go to your Mac system preferences and enable file sharing for the concerned user using ftp. You can then use a program like FileZilla on your PC to access files on your Mac. It will ask you for the Mac user name password of the person who shared the files. Once you get in, you can just drag and drop files from the Mac to the PC.