external HD must be scanned by scandisk every time for Vista Backup

Hi,

[Vista32 Home Premium]

I use an external harddisk (Lacie USB) as my backupdrive.
It is formatted FAT32. (Yes fat32, I need the thing also elsewhere)

Every time I connect it to my computer it works great.
I get no warnings. Everything works OK.

Except when backup kicks in. The backup fails always with the comment
that my backupmedia contains errors and thus it cannot make a backup.

Then I check it (rightmous on Lacie HD->tools->check for errors) but
scandisk NEVER finds any error.
After that useless check I can backup.

Does anybody know why Vista backup thinks wrongly my exteral HD needs
checking every time?

TIA

Regards,
Erwin Moller