Use MCT to create bootable Flash drive with previously downloaded ISO

No, you can't use the MCT to do this. But you don't need to, it's easy to do it yourself.

First prepare the USB. It needs to be at least 8GB. It needs to be formatted with Fat32 and (to allow it to boot on legacy bios systems) its partition should be marked as Active. You can usually use Disk Management to mark it as active by right-clicking on it's partition.

Then just double-click on the ISO file to mount it as a virtual DVD drive, select all the files and folders you see there and copy them to the USB. This USB is now bootable and identical to the USB that the MCT would have created.

A few USBs are partitioned in a way that doesn't allow you to mark it as active. If so, use Diskpart to prepare the USB, starting with the CLEAN command to remove all previous partition info from the USB (you can run Diskpart by typing DISKPART in a command prompt or in the WniKey+R run box). In this example the USB is disk 1, use LIST DISK first to find the number of yours.

Code:

Microsoft DiskPart version 10.0.16299.15
 Copyright (C) Microsoft Corporation.

 DISKPART> LIST DISK

   Disk ###  Status         Size     Free     Dyn  Gpt
   --------  -------------  -------  -------  ---  ---
   Disk 0    Online          465 GB      0 B
   Disk 1    Online         7634 MB      0 B

 DISKPART> SELECT DISK 1
 Disk 1 is now the selected disk.

 DISKPART> CLEAN
 DiskPart succeeded in cleaning the disk.

 DISKPART> CREATE PARTITION PRIMARY
 diskPart succeeded in creating the specified partition.

 DISKPART> SELECT PARTITION 1
 Partition 1 is now the selected partition.

 DISKPART> FORMAT FS=FAT32 QUICK
   100 percent completed
 DiskPart successfully formatted the volume.

 DISKPART> ACTIVE
 diskPart marked the current partition as active.

 DISKPART> EXIT

ricardobohner said:

...I remember that in the "Microsoft way" for creating a bootable instalation media in Windows 8 there was included one more step...
CD d:\boot
Bootsect /nt60 e:
After that you could copy the instalation files to the USB flash drive. Is this step not necessary anymore?

Actually the MS way seems to be the other way round, first copy the files to the USB then use Bootsect...

TechNet | Step-By-Step: Prepare USB Media to Install Windows 10 (April 2015)

As it happens, last week I made a Win 8.1 install USB by copying the files from a DVD that itself had been burned from an ISO made by the Win 8 MCT. I didn't use the Bootsect command and that boots fine on my legacy bios systems.

Apparently...

bootsect /nt60 sys
The bootsect command as used above will update the volume boot code on the partition used to boot Windows to BOOTMGR, the one that's compatible with Windows Vista, Windows 7, Windows 8, Windows 10...

How to Use Bootsect /nt60 to Update the VBC to BOOTMGR

...so the fact that my USB booted without being updated suggests it's a redundant opperation.

I don't have a uefi system to test my USB on, perhaps @NavyLCDR could comment on that?

After preparing the boot media with diskpart go to the source files (let's say d: are the source files and e: is you flash USB drive)

CD d:\boot
Bootsect /nt60 e:

You only need this if the master boot code in the USB is corrupted or in some case, you use an old USB stick with out of date firmware and unable to boot.

FYI, After you prepare the USB stick ie.

1. Format with FAT32 if your UEFI firmware is prior to version 2.0, else you can format with NTFS.
   NOTE: Format with NTFS will remove the 4GB restriction so you won't have any problem with install.wim/install.esd exceeding 4GB file size.
2. set Active if you are planning to install Windows using MBR disk type else you don't need to.
3. Instead of mounting and copy. You can extract the content of the ISO file using WinRAR/WinZip/7-zip
   OR You can also use command line with 7-Zip:
   7z.exe x Windowsx64.iso -o"F:" -y where F: is the drive letter of the USB
   NOTE: step 1 & 2 are needed for first time to prepare the USB. Any subsequent update, only need step 3.

Ricardo,

I agree with the others and would like to add one extra ditty.

Once you have made your Windows 10 installation disk from this ISO on this occasion, you can in the future delete all files then copy across the contents of a new ISO [by, as the others said, mounting it or just opening in WinZip etc].
- You do not need to start from scratch with DiskPart every time you want an updated installation USB.
- I have been doing this since about Ver 1511 and my installation disk works well for clean installations & repair installs.
- I go a bit further and hide the contents so that I can also use the install USB for other important files without risking accidentally overwriting any of the Windows 10 installation files. The install USB is perfectly happy to be used in this way.

Denis