This is a challenge a beginner faces when trying to copy data between Mac and Windows. Some of your USB drives may appear on Mac as read-only system. These are NTFS formatted disks.
Although Mac can natively write NTFS, this requires a system hack. It is also quite dangerous, as the drive can be easily corrupted. There are two options to use your USB hard disks between Mac and Windows:
1. Format your drive as ExFAT. This file system is supported on both Mac and Windows 7/8, it is fast and reliable. The only problem is that on Windows XP ExFAT requires installation of an additional software.
2. Use NTFS drive with an additional driver. There are multiple choices. If you are looking for software free of charge, you can try OSXFUSE or NTFS 3G. I personally would recommend the first one. If you do not mind spending money for MS file system support, you may try Paragon NTFS ($19.90) or Tuxera NTFS (EUR 25.00). Both paid options provide you a trial period to see if you really need that piece of software.
Personally, I have used to run some NTFS support, but eventually started to use ExFAT only. That would be my sincere recommendation, leave NTFS to Microsoft fans.
