Time machine

Just set up time machine, backing up to a 500GB Seagate external desktop hard drive. Problem is first time I turned the mac back on the icon for the drive selected had lost the time machine logo, it wont back up and the message reads 'drive not available'. Doubled checked the preference settings and it shows the correct drive all be it without the correct icon. Used select disc and choose the seagate drive again, icon re appeared and all works fine. If I re start it does the same again..........any ideas.

Cheers
Dale

Details
2GHz Intel Core Duo
Mac OS X 10.6.2
Seagate 500GB USB Desktop Hard Drive.

Is the drive turned on when your Macintosh powers up? You might want to try having it already going when you turn it on or restart or whatever.

Not a lot I can do about that, its permanently plugged in and powers up when I turn the mac on !
Dale