Format type for my HDD

The disadvantage is that you cannot write to it from the Mac side without buying a software package.

The reason you cannot initialize it as HFS+ is because you are not selecting the drive properly for initialization in Disk Utility.

1. Connect your drive and launch Disk Utility.
2. Click on the drive, and NOT the partition that is below it and indented. This brings up a new tab called Partition.
3. Click on Partition and from the pull-down menu choose 1 partition (or more). If you plan to boot from this drive you must select GUID partition table from the Options button. If you plan to boot from a PPC mac, you must choose the Apple table.
4. Select HFS+ with journaling enabled and initialize.

NTFS Limitations

• Max file size - 2^64 bytes (16 EiB) minus 1 KiB
• Max number of files - 4,294,967,295 (2^32-1)
• Max filename size - 255 UTF-16 code units
• Max volume size - 2^64 − 1 clusters
• Allowed characters in filenames - In Posix namespace, any UTF-16 code unit (case sensitive) except U+0000 (NUL) and / (slash). In Win32 namespace, any UTF-16 code unit (case insensitive) except U+0000 (NUL) / (slash) \ (backslash) : (colon) * (asterisk) ? (Question mark) " (quote) < (less than) > (greater than) and | (pipe)

HFS+ (Mac OS Extended) Limitations

• Max file size - 8 EiB
• Max number of files - Unlimited
• Max filename size - 255 characters (255 UTF-16 encoding units, normalized to Apple-modified variant of Unicode Normalization Format D)
• Max volume size - 16 EiB
• Allowed characters in filenames - Unicode, any character, including NUL. OS APIs may limit some characters for legacy reasons

the limitations of NTFS are nothing to get worried about but HFS+ is a better filesystem in terms of limitations

Mac OS X can only read NTFS but if you install "Mac FUSE + NTFS3g" (open source) or Paragon NTFS for Mac OS X (license) you can read/write to NTFS. but if you dont have to use NTFS i would strongly suggest you use HFS+ but this means you will have to reformat.