Got error "Your bootable USB drive could not be created." while creating Windows 7 bootable USB in Boot Cam Assistant

To install Windows 7 on Macbook Pro, you need Boot Cam Assistent app. It's built in OS X. Note that I'm using Macbook Pro 13'' Late 2011 and Max OS X Mountain Lion 10.8. I'm using Boot Cam Assistant to create a bootable Windows installation USB so I don't need Windows Installation DVD.

Note that before starting Boot Cam Assistant, I enabled the option to create install USB in Boot Cam Assistant (BCA) as following:
- Go to Finder -> Applications -> Utilities
- Right click on Boot Cam Assistant.app and select Show Package Contents
- Make a backup copy of Info.plist file then open it and change the key PreUSBBootSupportedModels to USBBootSupportedModels and save

However, when I clicked on the Create button in the last step, it said "Your bootable USB drive could not be created. An error occurred while copying the Windows installation files.".  Here are what I tried to fix the issue:

1. Unmount the Windows installation ISO file. I might clicked on it once and It was mounted. I tried to unmount it from both Finder and Disk Utility and the use Boot Cam Assistant to create it again but no luck. I've noted that the people on the net who got their problems solved using this way got the error message immediately after they click on the Create button. For me, the message showed up about 10 minutes after clicking on the Create button.

2. Someone said it might be the problem with the ISO file. They solved it by using Disk Utility to generate a new one from the current ISO file. They run the command:

hdiutil makehybrid -iso -joliet -o new_win7.iso /Volumes/DATA/Softwares/OS/Windows/Windows_7_Sp1_Ultimate_x64.iso

Unfortunately, it still didn't help.

3. I tried to use Windows USB/DVD Download Tool (https://www.microsoft.com/en-us/download/windows-usb-dvd-download-tool#installation) to create a bootable USB instead. It's Windows program so I run Windows 8 as guest OS in VirtualBox and then used the tool to create a bootable USB. But, the tool couldn't create it successfully. It had another error. It told me to check my USB drive and the Windows installation ISO file. Damn!

4. And then I tried to install Windows using the ISO file in VirtualBox, it failed while it was trying to copy the installation files. It said some files were corrupted. Hence, I thought it might be the problem with that file so I downloaded two other different ISO files (from piratebay website) and used them with Boot Cam Assistant. I got the same error for the second ISO file but I successfully created the bootable USB using the third ISO file. I don't know why and I don't care.