Q: Unable to install Windows 10 from USB drive on MacBook Pro mid 2015 with OS 10.11.4

Check the partition map of the drive. It must be GUID. The drive is probably Master Boot Record.

Even if it already is GUID, I had the same message the first time I went to install Win 10 on a drive that I knew for a fact (and verified) as being GUID. Win 10 still wouldn't install until I completely repartitioned the drive.

Open Disk Utility. Highlight the physical drive name at the far left. The partition map should be shown a the lower right. That in OS X Yosemite and earlier. On my iPad at the moment and can't be sure where it's shown in El Capitan.

Apologies for barging in, Kurt Lang.

Please see Boot Camp: "We couldn't create a new partition or locate an existing one" alert during Windows 8 install - Apple Support .

All external storage should be disconnected. If you are using a USB HDD, please use a 8-64GB USB Flash drive. SD Cards are not supported for Windows installation because there are no drivers in W10 for such devices.

2015 Macs do not use a USB, but an internal partition called OSXRESERVED. Please see https://help.apple.com/bootcamp/assistant/6.0/#/bcmp173b3bf2 for reference.

Apologies for barging in,

No problem at all, Loner T. Extra info is always good.