Have you read this thread?
http://talk.maemo.org/showthread.php...6&goto=newpost

Yeah, I know, it's huge. There might be some help. Have you installed previous minor update 44? You can not update to 51 before you have 44 installed.

Also, are you using the latest versions of Ovi or PC suite?

Check also, that the phone is in PC-suite mode when you connect it to PC.

I have used PC-suite on windows laptop and it did recognize N900, but only when I used specific USB-port for connecting the phone. If the phone was in any other port, it was not recognized. So maybe you can try to connect to another USB-port. Also, do not use an USB-hub in between your phone and PC if you have one.