editing from a full hard drive, bad?

External eSata or USB3 are both fast enough for video editing... (or Firewire?) so I hope your external isn't USB2

I don't use an external for editing, but I do have multipe 'bare' drives I use for backups... I have a Star Tech base that I connect to USB3 and rotate my bare drives

http://www.amazon.com/StarTech-SuperSpeed-Docking-Station-Cooling/dp/B0055PL2YI or http://www.newegg.com/Product/Product.aspx?Item=N82E16820147511

My two cents:

my question.  is it bad to put both on the 1TB?  leaving only 10-20% space left on it? wil i have problems with playback, premiere going slow etc?

Is the drive a hard disk or an SSD? If a hard disk, best practice is to work with the drive less than 60-70% full. Pushing it out to 80-90% full will impact drive performance. Will this be problematic with Premiere? Likely, but you may get away with it. But is it good practice to work with the hardware at its marginal limits?

would it be better advised to buy a second 1TB drive and keep both of them just under half full for optimum performance?

I would say yes, and you'll also have the perfomance benefit of spliting the media load across two drives. My recomendation, however would be to get 3 more drives - one to split the media on, and two to make backup copies of the two drives you will be using in case one of them fails.

MtD