Cannot access client mydocs folder

By mounting a clients booting hard disk in my desktop system as a secondary drive, I am usually able to access their documents folder for backup when their OS fails to boot. Occasionally I get an error that says “Access is denied” even if I create a user on my system with the clients user name and password. The properties for the denied folder shows read only and 0 bytes. The user professes not to have turned on encryption. Any Ideas? Thanks.