The MAC can read but can't write to an NTFS drive.
On a PC you can format your drives as FAT32 or NTFS. The Mac can read and write FAT32 but can only read NTFS. The PC can't read or write a disc formatted in one of the Macs native formats. The best bet for you is therefore FAT32 since they both can read and write to the drive without special programs.
You need to have a backup drive anyway and they are very cheap now so buy another external drive and format it FAT32. Copy all of the files from the NTFS drive over to it. Once you are confident you have a good copy, format the NTFS drive as FAT32 and copy the files back to it. Now you have 2 drives with the same files that both computers can read and write.
On a PC you can format your drives as FAT32 or NTFS. The Mac can read and write FAT32 but can only read NTFS. The PC can't read or write a disc formatted in one of the Macs native formats. The best bet for you is therefore FAT32 since they both can read and write to the drive without special programs.
You need to have a backup drive anyway and they are very cheap now so buy another external drive and format it FAT32. Copy all of the files from the NTFS drive over to it. Once you are confident you have a good copy, format the NTFS drive as FAT32 and copy the files back to it. Now you have 2 drives with the same files that both computers can read and write.