I have USB 3 external HDD hooked up to an iMac. It seems like if I have the external drive, the initial boot-up takes several seconds more. external drive has no OS X installation and Start-up Disk, in System Settings, is set to internal SSD and no other drives even show as an option. Is this issue caused by ANY external drive? or could this vary between external drives? I am using Lacie's 3TB HDD.