Windows – Does Bootcamp support 64 bit version of Windows 7

Is your Windows 7 64-bit? If so, you might want to install the drivers in the Mac OS X DVD.

D:\Boot Camp\Drivers\Apple64.msi

The (fixed) Pre-Installation Environment (boot.wim) of your Windows_7.iso doesn't contain the necessary drivers to use the USB3-interface of your Late 2013 Macbook Retina. So a "non-standard Windows7.iso" including "additional drivers" without injecting/implementing them into the boot.wim won't help much either. Btw BOOT (X:) is not your install drive but the mounted boot.wim.

To circumvent limitations (new drivers, new hardware) Microsoft uses the $WinPEDriver$-feature to provide drivers at installation time.

Apple Boot Camp uses this method to provide the necessary drivers with the boot medium - your external USB drive.

Please check if you meet the following requirements:

1. Backup of your entire hard disc

2. Windows 7.iso Home Premium x64, Professional x64, or Ultimate x64 (these three are us-en digitalriver isos and here is a full list of all images available there). Please use Microsoft Software Recovery to download the proper Winodws 7 release.

3. Formatting of the USB-device (according to Apple):

   If the size of the disk is 32 GB or less it should be MS-DOS (FAT).

   If the size of the disk is over 32 GB it should be ExFAT.

   Try to use an USB thumb drive (size 8 GB or bigger). If you don't have a thumb drive, try to use an USB hard drive smaller than 2 TB. ExFAT formatting with Apple disk utility.app often doesn't work (means: formatting ends in FAT32 instead of ExFAT) and according to different sources a volume size limit of 2TB exists for MS-DOS (FAT) (=FAT32).

4. Boot Camp Support Software version: 5.1.5640

   download link: Latest Boot Camp Support Software

5. File/Folder structure of the root of your boot device (your external USB-drive):

6. If your boot device doesn't contain $WinPEDriver$, AutoUnattend.xml and BootCamp please download and extract BootCamp5.1.5640.zip and copy the contents of the resulting BootCamp folder to the root level of your install drive.
7. If your install drive already contains those files and folders then check if the USB drivers are present. (Paths: /$WinPEDriver$/IntelxHCISetup/Drivers/HCSwitch/Win7/x64/ and /$WinPEDriver$/IntelxHCISetup/Drivers/xHCI/Win7/x64/). If they aren't present the wrong Boot Camp Support Software package has been chosen. Delete $WinPEDriver$, AutoUnattend.xml and BootCamp and continue with step 6.

Now the installation should work.

If the installation still doesn't work a more time consuming way has to be chosen (modifying the boot.vim) and i will write a second answer.

P.S. Did you already check this apple.stackexchange question/solution? Especially the comment of user funkylaundry May 18 at 16:38to the 1st answer: "the key was to entirely remove the bootcamp partition and then add it again"