According to http://support.microsoft.com/kb/926185, Vista uses shadow
copies to store backups and restore points, and dual-booting with XP
makes you lose those.

Suppose I back up to an USB disk: it wouldn't seem logical that a
shadow copy would be used there as well, but just to be sure: is it?

Because if it is, it looks like connecting that USB disk to an XP
computer would make me lose my backups.


I've no safe feeling in this regard, because I noticed that when I
create a full system backup to that USB disk, the backup file created
there is only a fraction of the used space on my system drive, even
after subtracting the size of the pagefile.

After installing Vista Ultimate and some software, about 60G was used
on my C: drive. A full backup is supposedly only taking up 15G in the
WindowsImageBackup tree on the USB disk.