I have a Flash Drive and an external usb working just fine but with my ext/int usb drive Win 7 keeps forgetting that it is formatted. Manage and minitool partition both see it and all folders read/write - all partitions are NTFS- drive is 1Tb Hitachi with its own power supply. I have spent days on the net trying to esolve this.
This may sound like a stupid question but have you fully updated your windows with windows update? I think I might remember there being an update to patch an issue with large usb drives.