when you use data transfermode, you don't need pc suite at all. data transfer mode equals mass storage mode (google mass storage mode to learn more). mass storage mode is such that it freezes your phone until you do what you need to do (transfer data from or to phone) and then you release it. You should use the safely remove hardware in PC option when you use data transfer mode.
If you want to use pc suite, select pc suite mode from phone, then the phone does not freeze during usage with pc.