External Hard Drive Now Read Only

From My Computer, right click on the drive and select Properties. Look for the Security tab and see if the permissions there are limited to Read-Only and see if you can allow full permissions to write, edit, whatever else. Don't know if it'll be that easy, but it's worth a shot.

Also along those lines, you may have to be logged in to the computer as the Administrator to effect such changes.

does the drive have a write protection switch on it?

google results say that macs can't write an ntfs formatted drive, only read it. that may be your problem