After I replaced the original HDD with a 120GB Seagate, I could no longer FTP my Xbox. I used Superdisc 3.6 to format the drive and get all necessary programs installed. Everything but the FTP works just fine; I can play backup games, all programs work, no problems with the DVD or Xecuter 2 Lite chip...
The FTP is working with the old HDD, so it´s not something with my settings on the PC. What do I do to fix this?

Are you using EvolutionX dash as your FTP server on the xbox?
You must have different settings - check the System Settings in EvoX to see what the network section say, and change to match your old settings if necessary.

well if your old harddrive worked it cant be your pc unless your new xbox's ip is different. I had a similar problem though. on your xbox go into system management-system settings. make sure your use static ip is yes and the two under default gateway- DNS & DNS make sure they are 0.0.0.0 and then make sure you go down to save and exit. reboot and see what happens.