How to Install / Bootcamp Windows 10 on a mid-2011 iMac using USB
Apple has released support for bootcamping Windows 10, but only on 2012 Macs and later. Despite not being supported. it is possible to install Windows 10 on earlier iMacs and it seems to run quite well.
IMPORTANT: Unplug all external and physical hard drives (where possible) that you won't be installing to to avoid accidentally erasing them. Also make note of which drives and partitions remain (e.g. System and Storage hard drives), and be super careful to not erase the wrong one.
RECOVERY: If you nuke your machine, restore your time machine backup. Instructions here.