backing up files

I have time machine backing up my computer but I only want my photos and video backed up. The external back up is full and now files are dropping off it. Should I just get another external hard drive and drag my photos and video over. I have one I used for windows and my mac did take the files over but would not let me store any from my mac on it.

A new large external hard drive would be best. Always keep your backed up files separate if possible. Mixing Mac and Windows files on the same drive is not good practice and does not allow the use of Time Machine since the drive has be formatted as HFS+. You could partition the external hard drive but again, not good practice.

There is a lot going on in your post, grannyem. If TM is full, it will start dropping older backups to make room. If you want not to do that, you'll need to make another drive available for TM. But if all you want to do is copy photos and video over to the Windows formatted drive, Paragon NTFS (search for it) will let you do that from the Mac. On the other hand, if you don't need the windows stuff anymore, you could reformat that drive to a Mac suitable format using Disk Utility and then copy/past the photos/videos. If all you want TM to backup are photos/videos, open TM preferences in System Preferences and under Options, add to the list what to EXCLUDE from the backup. In your case, that would be everything but the photos/videos. As chscag said, the best is a large ext HD added to your system for either backups or photos/videos, or both.