Time Machine problem

I'm trying to use an external usb drive for time machine. With the drive hooked by USB to my MacBook Pro time machine works fine with it. I don't want to keep it hooked up all the time. I have the USB external drive hooked to my Windows 7 desktop and have it shared (yes I have Macdrive 8 so my PC will see it) and can see and write to the drive from my MacBook wirelessly. But if I try to use that drive for Time Machine (I can choose the right drive because I configured to show unsupported network volumes) I get the error: The Backup Disk image "thenameofmyvolume.sparsebundle" could not be created (error 45).
Any help would be appreciated.

Sounds like you've done step 1 & 2 of this work around. Still have a way to go.

Thanks for the help. That was excellent.