Why can't I put files on removable drives?

I'm using a Mac OS X Version 10.3.9 with PowerPC G4 processor and a 1MB cache, and I can't put files on flash drives, removable hard drives, etc. The only thing it will allow itself to put files on is my PSP (Playstation Portable). I need to figure out why it won't put all my files on my brother's 100GB removable hard drive so I can finally use a new computer I got, but every time I try and put something on it, it says "The Item "untitled" cannot be moved because "RemovableHardDrive" cannot be modified." I then went to Get Info for the hard drive, and it says the format is Windows NT File System, it says it hasn't been indexed, and that for Ownership and Permission, it says "You can only read" I used to be able to modify it about a year ago, but I can't anymore, and I haven't done almost anything to it other than put songs and pictures on it. Can someone explain to me why it isn't working, and how I can fix it?
P.S. It also is an extremely slow computer, both on and offline, I'm not sure if that would help someone out to know, but I figured I should at least put that out there. (I once has to shut it down because it stopped responding while I was editing a text file)

The NTFS file system is "read only" from a Mac. The only way you can write to that drive is by using a third party application. Paragon NTFS can be purchased from Paragon software for $39.95. Link

Regards.

Unfortunately, the current versions of Paragons product won't work for him. Neither will the current ntfs-3g, as they both require Tiger 10.4 or higher to work (paragon requires 10.4.6 or newer)

BUT - you may have an option - it looks like there was an older version of Paragon (and possibly older version of ntfs-3g, but it'll be harder to find I think) that was written for 10.3 - you might want to contact them (paragon) to find out if its still available.

Since the op has 10.3, choices are a bit more limited =( I only did a REALLY quick search, but I didn't see anything current that explicitly covered 10.3