log says successful backup...but no backup can be found on drive

Log says "successful" but no data can be found on back-up drive

Once a week I have scheduled acronis to do a data back-up of "my documents" folder only. EVERY TIME at the end...the log says "Successful backup". HOWEVER, one out of three times no actual backup can be found on the backup drive. During the backup process BOTH my home computer and back-up drive are chugging away for 4-hours. When I compare logs from each backup they are the same. Where is all that data going? Why does it work sometimes and sometimes not? How can it be determined to be a "success" if no data shows up on the back-up drive? THANKS. --Chris--