re: Why do BackupRead and BackupWrite require synchronous file handles?

Certainly introducing a breaking change in the backup world is the worst kind of pain. If backup is broken, that leads to all kinds of user hurt.

#ifdef _x86_

// synchronous file handles only, please

BackupRead(...);

BackupWrite(...);

#endif

// works with asynchronous file handles

// (and synchronous ones too, of course)

BackupRead2(...);

BackupWrite2(...);