Changing the permissions of a external hard drive without backing it up.

Okay so, I have a MacBook Pro 13' Mid-2012.

My problem today is with an external hard drive I've had for a while now. Usually with this hard drive, I would just watch movies that my father or friends of family put on it, so it was never really a problem. But today, I noticed I'm running out of storage space on my internal hard drive so the idea came to me that I should use that external one as an external storage unit (mainly for my iPhoto library and movies/tv shows).

I went to drag a photo onto the ex hard drive and it did not work, then I tried deleting something off the ex hard drive --there wasn't even the option to.

After 2 hours, I realised the permissions was set to Read only and it was formatted as NTFS (probably from my fathers Windows PC). I do know how to change it to Mac OS Extended, through disk utility and the partitions etc, but I read that I should back my ex hard drive up first because if I wanted to change it to Mac OS Extended I would have to erase it, the problem here is, I don't have any storage to back those things up.

I only have 23.77GB free of 498.88GB and the size of the things on my hard drive is 147.78GB.

Help me !!!!!!!

Btw, this is my first time asking a question in this community.