Cannot write to external HD anymore on mac

So I have a Western Digital 1 Tb hard drive that I bought a year ago. I have used it with both my macbook and macbook pro, and have been able to read and write to both of them. I was also able to read and write on my PC as well. I plugged my external HD into my macbook pro today and now I am no longer able to write to it, only read. I have not changed any of the settings at all, and when I look at the device properties, it says it is in NTSF format, which is impossible because I had put over 900GB of data onto this hard drive using my macbook and macbook pro. Yesterday I plugged my hard drive into my Samsung TV directly, to watch movies and there was no issue, but I noticed there was no "safely eject device" option anywhere. I know that if the TV is only reading and not writing, that an eject button is not necessary, but can the TV itself switch the format of my external hard drive?!? How on earth do I go about fixing this without having to re-format why external hard drive?

Something changed the file format on your WD drive. Whether it was the Samsung TV or your PC is unknown. Using the external drive as a "do anything" drive is not a good idea anyway. Keep one drive for your Mac and another for the PC. And maybe even another for the Samsung. You can reformat the drive using Disk Utility from your Mac.

I can reformat the hard drive yes, but that will make me lose all 950 Gb of info right? as I dont have enough space on any of my comps for all that data

Yes, reformatting the drive will result in the loss of all the data. If you need the data from the drive, attach it to your PC which can read and write to NTFS, and then copy it to another drive. After you extract the data, you can format it back to the Mac format. However as I said before, it's best to have a separate drive for the Mac and one for the PC.