I have a problem with Yosemite that I never previously had with Mavericks. I connect my mac to an external monitor (via thunderbolt) and use this monitor to use my mac (i don't use the iMac screen). I do this because I share the monitor, keyboards etc via a KVM switch. On Mavericks, the login screen either displayed on my external monitor, or both. I have mirroring set up so the mouse doesn't go over to the iMac screen (the iMac is out of viewing distance). Now on Yosemite, the login screen only ever shows on the iMac. It occasionally shows on the external monitor, but 9.9 times out of 10 it displays on the iMac only. Has anyone else encountered this, and if so, have you found a resolution? Thanks in advance.