mne len' smotret' v MSDN..
vozmojno, CFile vozrvatit ne bytescount v tom sluchae, esli chast' fayl-a sdelana LOCK kem-to (libo vladel'cem, libo so storony).
Vtoroy variant - toje ne uveren - esli fayly sdelany CreateFileMapping.

tuxlymi EXE-shnikami ne kidat'sya