To start off, you have not really said what iMac you have, so that adds a level of complexity to the problem.
I have a 2014 Retina iMac that happens to have a small SSD internal. I wanted to have a windows 10 bootcamp sitting on an external drive as I don't have a whole load of space internal. I have a 3tb Lacie Thunderbolt drive that I partitioned 100% over to HFS+ that I use to put all my non-speed critical files onto. I did install a very small Windows 7 bootcamp on the internal SSD (32gb) but it really isn't large enough to run anything off and doesn't like the fact that I have 16gb ram and there is not enough drive space for a pagefile.
I went out and purchased a USB3 external drive bay based on the JMicron JMS567 chipset that is supposed to work well. I put in a 120gb SSD that I got on sale and booted into Windows7 bootcamp. I downloaded the Windows 10 iso image from Microsoft and ran WintoUSB (
http://www.easyuefi.com/wintousb/). This app will take your .iso image and make a bootable version of Windows onto the USB3 external drive. It worked perfectly fine for me at first. The down side is that Apple doesn't seem to like you doing it this way as there really isn't a clean way to get Bootcamp drivers installed. I tried running the Win8 version that you can download off the internet and then allow it to update itself. It mostly works except the upgrade crashes at the end and it doesn't realize that it is now up to date.
I don't know if it's my iMac at fault, but after about 3 weeks the USB3 drive stopped working in you can select the windows boot by holding down the option key on startup and selecting the USB drive, but the windows would not boot from it rather reset the iMac. It worked fine if I used an old 500gb hard drive that was noisy with a USB3 dock.
I went back to the drawing board and bought a small Thunderbolt drive that was silent and gave it a try again. The WintoUSB app doesn't work with Thunderbolt drives. So I gave the microsoft media creation tool and got it to make a bootable USB install drive on a 16gb flash stick I had. It would boot perfectly fine as long as you plug it into the first USB3 port nearest the power supply. Windows 10 recognizes the wireless bluetooth keyboard and mouse fine. It will see the Thunderbolt drive, but it took a bit to convince it to install to the drive. I ended up formatting the drive as Fat32 GUID partition map then in the windows install, you select all the partitions on the drive and delete them, and then let windows re-create and format the drive. It works perfectly. Again, you are back to you can't really install the bootcamp drivers. I didn't even bother and just installed an updated display driver from bootcampdrivers.com and those work fine. My Win10 works fine now.
Just letting you know it can be done, but not for the fiend of heart and not without quite a bit of perseverance.
