Time Machine backs up differently to a "local" drive that is connected directly to a Mac than it does when the drive is connected to a Time Capsule or AirPort Extreme as a "network" drive.
All of the existing backups on the drive that you have now were made when the device was acting as a network drive.
If you now have the drive connected directly to your Mac, it is functioning as a local drive, so Time Machine will see this as a new drive, one that it has not backed up to before......so it will make a new complete back up of your Mac and then move forward with incremental backups after the first "master" backup is done.
If you really don't need all the old backups and don't mind starting over again, you could erase the drive....which will only take a minute or two...and then start over again with new backups. This is probably the most efficient thing to do, and you will have the entire drive space for backups going forward, but I understand that it is not easy for some users to erase their old backups.
Or, you can keep the old backups and start a new backup using Time Machine. The downside to this is that you have two different backup sets on the same drive, which will take up quite a bit more space than a single backup set, so space may be limited for your new backups.
