Hello,
BC Classic does not have a separate "Encrypt" right-click menu. To encrypt a file with BC Classic, you can simply copy or move the file to the Boxcryptor Classic disk in your Finder.
If the problem with the "disappearing" drive persists, please create a debug log and send it to us at support@boxcryptor.com. You can create the debug similar to the regular Boxcryptor software (just replace "Boxcryptor" with "Boxcryptor\ Classic"):
https://support.boxcryptor.com/display/BCOSXEN/How+to+create+a+Debug+log
Best regards,
Robert