Q: using bootcamp on imac late 2015 and el capitan ti install windows

You cannot install Windows 10 on a USB flash drive or an external drive using a Mac.

How to install Windows using Boot Camp - Apple Support

I think you are missing the point. If you did all that Windows 10 would already be installed on your Mac. If you want to create a bootable Windows 10 USB flash drive you have to do it by running your Windows 10 install then creating it from there.

You're welcome.

With the USB connected to the Mac, from OS X Terminal, please post the output of

diskutil list

Run Applications -> Utilities -> Terminal, and then type the highlighted command. You should get output similar to the following.

diskutil list

/dev/disk0 (internal, physical):

  #:                      TYPE NAME                    SIZE      IDENTIFIER

  0:      GUID_partition_scheme                        *512.1 GB  disk0

  1:                        EFI EFI                    209.7 MB  disk0s1

  2:                  Apple_HFS OSY-MBP13              380.3 GB  disk0s2

  3:                Apple_Boot Recovery HD            650.0 MB  disk0s3

  4:      Microsoft Basic Data BOOTCAMP                131.0 GB  disk0s4

/dev/disk1 (external, physical):

  #:                      TYPE NAME                    SIZE      IDENTIFIER

  0:    FDisk_partition_scheme                        *16.0 GB    disk1

  1:                DOS_FAT_32 WININSTALL              16.0 GB    disk1s1

 

diskutil info disk1

  Device Identifier:        disk1

  Device Node:              /dev/disk1

  Whole:                    Yes

  Part of Whole:            disk1

  Device / Media Name:      Cruzer Glide

 

  Volume Name:              Not applicable (no file system)

 

  Mounted:                  Not applicable (no file system)

 

  File System:              None

 

  Content (IOContent):      FDisk_partition_scheme

  OS Can Be Installed:      No

  Media Type:              Generic

  Protocol:                USB

  SMART Status:            Not Supported

 

  Total Size:              16.0 GB (16008609792 Bytes) (exactly 31266816 512-Byte-Units)

  Volume Free Space:        Not applicable (no file system)

  Device Block Size:        512 Bytes

 

  Read-Only Media:          No

  Read-Only Volume:        Not applicable (no file system)

 

  Device Location:          External

  Removable Media:          Yes

  Media Removal:            Software-Activated

 

  Virtual:                  No

  OS 9 Drivers:            No

  Low Level Format:        Not supported

rflombardi wrote:

 

Last login: Sat Sep  3 12:50:36 on ttys000

Ron-F-Lombardis-Mac-Pro:~ rflombardi$ diskutil list

/dev/disk0 (internal):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                         24.0 GB    disk0

   1:                        EFI EFI                     314.6 MB   disk0s1

   2:                  Apple_HFS OS X Base System        23.6 GB    disk0s2

/dev/disk1 (internal, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *1.0 TB     disk1

   1:                        EFI EFI                     209.7 MB   disk1s1

   2:                  Apple_HFS Macintosh HD            999.3 GB   disk1s2

   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3

This is a Fusion drive which you have 'broken'. Can you explain why?

/dev/disk2 (external, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *32.0 GB    disk2

   1:                        EFI EFI                     209.7 MB   disk2s1

   2:                  Apple_HFS Untitled                31.7 GB    disk2s2

This is incorrectly formatted/partitioned. It should look as follows...

/dev/disk1 (external, physical):

  #:                      TYPE NAME                    SIZE      IDENTIFIER

  0:    FDisk_partition_scheme                        *16.0 GB    disk1

  1:                DOS_FAT_32 WININSTALL              16.0 GB    disk1s1

It needs to be formatted as follows...

Please unmount all ISO images - disk3 and disk4.

Disconnect the external USB flash drive. It is not needed as documented in How to install Windows using Boot Camp - Apple Support -

• For most Mac models, you also need a blank 16 GB or larger USB flash drive
  You don't need a USB flash drive for MacBook Pro (Retina, 13-inch, Early 2015), MacBook Pro (Retina, 15-inch, Mid 2015), MacBook Air (13-inch, Early 2015), iMac (Retina 4K, 21.5-inch, Late 2015), iMac (21.5-inch, Late 2015), iMac (Retina 5K, 27-inch, Late 2015), iMac (Retina 5K, 27-inch, Mid 2015), and Mac Pro (Late 2013). These models use the internal drive to temporarily store what you need to install Windows.