Ok for starters that is WAY to little free space, no matter how big the drive is. as rod said, OS X likes to have 10-15% of an HD free, 3-5% of an SSD (I don't know why the difference in media, it just is). So unless your HD is tiny, 8GB isn't enough free.
This was wrong:
No it's not TM, but a Seagate external.
Time Machine (which is software) is NOT Time Capsule (which is hardware) and it's important to know that and keep it straight. So your answer was not logical. What Rod said was, "Roger I hope when you are referring to "Backup Files" you are not referring to Time Machine backups." The reason for that statement was, as he said, that the way TM backs files up you should not use Finder or any other tool than TM to reach in and delete a backup. It messes up TM and the file structure on the drive TM is using as a device. So, you have a Seagate external drive and I suspect you are using Time Machine to make the backups. And because of that, you should not be going to that Seagate directly, but through Time Machine to do deletions. Now that you've bypassed that way, the directory of the drive is probably messed up and TM is confused. And the way to restore from the trashcan is NOT to drag the file anywhere, but to right click and choose Put Back and let the trashcan do the work properly. That is particularly so with TM backup files. They aren't really what you think they are, so manipulating them that way just makes it worse.
At this point you may be in the position of having to reformat the external (losing all your TM backups in the process) or getting a new external drive and reinitiate whichever of them you choose as a TM backup drive, let TM make a good backup again and then trying again to delete files from your internal drive. You may even be in the position where some Terminal work may be necessary to clean things up, but the commands in Terminal are pretty powerful and you don't seem to be a power user, so I would recommend you go very, very carefully into that area. Maybe a power guru like Cradom can chime in here with some command line entries that may help you empty your trash through Terminal.