First, some excellent user videos for migrating to Thunderbolt or USB3.0 Drives:
There are some general considerations for running Windows Boot Camp from an external drive:
1. The Windows operating system must support EFI booting. Windows 7 is not compatible but Windows 8/8.1 and higher support EFI booting.
2. The external drive's logical sector size must match the source drive's logical sector size in order for Windows to restore correctly to the external drive. See this article for more information about advanced format drives, emulation and how to determine a drive's logical sector size. The 2015 MacBook with USB-C port is the first Mac to use advanced format 4K byte sectors natively and may be a sign of things to come. External drives could be either 512 byte emulation or 4K, so make sure to check the sector size of the external drive matches the internal drive.
3. Full EFI compliant firmware on the Mac computer. With some exceptions, late 2013 Macs and newer are UEFI 2.0-compliant.
4. Device drivers. Migrating Boot Camp from HDD to SSD (or from any internal drive to external drive) will require different storage drivers to be installed for Windows to boot successfully. We advise creating a Winclone image of Boot Camp before making any changes and try restoring and booting that from the external drive. If it blue screens on startup, it means that the old storage drivers aren't compatible and need to be removed before booting.
Plan B in that case is to go back to the source Boot Camp and run Sysprep to remove the device drivers, create a new Winclone image and restore that to the external drive. See our help document on migrating Boot Camp for more details about using Sysprep.
5. External drive controller/interface must be able to boot Windows. This one is more difficult to assess as some interfaces work and other do not, so there will be trial and error to find one that works. In our testing, Thunderbolt interfaces are very reliable for external Windows booting. The downside is they are more expensive (and some new Macs don't have a Thunderbolt interface). Not all USB3 drive interfaces will work. The enclosures that provide separate power are more likely to work than interfaces that rely on bus power.
6. Drive letter change with twin Windows. There may be issues when two identical Windows file systems are present at the same time. Windows will relegate one of the systems (usually the external system) to the next available drive letter, so instead of "c:/windows/system32/path_to_dll_files" for example, the external drive will use "e:/windows/system32/path_to_dll_files". Any services or applications that rely on fully qualified paths will fail or try using the resources on the other Boot Camp file system that is currently assigned to C:. The System IDs (SID) of the two Windows system will be identical and may cause instability with some applications.
The ideal workaround is to temporarily disconnect the internal drive so the original Boot Camp is not present when booting from the external Windows system. If that's not an option, then wiping out the original Boot Camp partition is the next best, riskier alternative but the Winclone image can be restored if something goes wrong.