windows - Can't write HFS+ in OS X

I've an external HDD of 500GB and in HFS+ format. It works normally on my Mac running OS X 10.5.8. After I plugged it into a machine running Windows, I found a vacant partition of 128MB and formatted it to NTFS. I used Paragon HFS+ for Windows to read and copy files.

Now I'm not able to write to that disk on my Mac. What is the problem?

Answer

this problem has been solved by booting in to the Lion Setup disk and using the disk utility to repair it, now it's working fine.