Time machine backup problem

[motox25]

Okay so I am having a strange time machine problem that has me stumped and I was hoping someone could help me resolve this issue. I had a powermac G5 and used a western digital USB external HDD to backup my mac with time machine. It is running the latest version of leopard. Half the time I would turn on the HDD, time machine would start to backup the computer normally. Then for some reason all my USB devices would stop responding (Apple keyboard, mighty mouse, western digital hard drive) and the computer would say that the device was improperly removed and the backup would fail. I then have to unplug and plug in my keyboard mouse and external drive to get them to work again. So I figured it was an issue with my old logic board or something with that G5. So I just bought a mac pro (specs in signature) and a new iomega external HDD and it does the exact same thing! It made it through an entire backup once but now almost every time I turn the drive on it does this.