No Internet when external hard drive is connected

Hello,

i have an external Hard Drive with two partitions, one for Time Machine Backups one for other stuff.

When i connect my external hard drive to my Macbook Air "13, the internet stops working.

I tried the same with some USB-Sticks, internet works well. But when i connect the hard drive again, internet gets so slow, can't eve access google.com.

Any ideas?