Yes I think you are correct in what you say in the first sentence. I just don't really understand, what is the solution, to keep it plugged in 24/7?
Internal drive and external are both the same size, 320 gig.
The estimated size of next backup is 314 gig.
Backups normally don't take long. I haven't done a lot of them mainly because of this problem. The first ever one took a long time, after that they didn't take long.
OK, if you have TM turned on but no external HDD connected it stores your backups on your internal HDD until you connect an external HDD. That means after even a day without the external HDD connected you've got quite a lot of data to backup.
You could keep your external drive connected all the time but that might be impractical as you'll have to lug it about with you. One alternative is to turn TM off until you can connect your external HDD, then turn TM on and select "Backup Now". Let TM do a backup or multiple backups whilst the external HDD is connected. When you disconnect the HDD turn TM off again, this will stop it storing backups on your internal HDD.
Of course you could use other backup software that gives you more control about when and what you backup - I'm thinking Super Duper or something like that, I use it as well as TM - belt and braces approach. Either way keep TM turned off until YOU want a backup.
You might have to delete some backups manually in order to get things back to a workable state and I think you need to look at just what you're backing up and exclude some stuff that doesn't need backing up. For instance if you've got original install disks for apps do you need to back them up from your HD?
You also need to find out what Disk Utility does and how to use it. HDD's do get corrupted and need fixing, Disk Utility will tell you if your drives need repairing. Go to the Apple Support pages online and learn about Disk Utility, you can screw your machine up with it so make sure you've educated yourself before doing anything. This might not make sense to you but I verify my disk and permissions once a month to make sure my machines are OK.
Stewart