The Plan I've been waiting to install the final release of El Capitan on an external HD, which I will then ship to an elderly friend who lives several states away (I already have Mavericks on another partition of the same external HD). He needs to continue using Snow Leopard as his primary OS. He refuses to risk trying to install a newer OS on an external drive by himself, and I don't blame him (I've been helping him for ten years). Catch No. 1 All or most versions of OS X older than Yosemite (I don't know about Yosemite itself) can be installed on an external HD while connected to one Mac, then that external HD can be used to boot other compatible Macs. El Capitan's required firmware update makes me doubt this is possible anymore, at least not without running the firmware update on the other Mac first. Am I wrong about this? Catch No. 2 The El Capitan DPs and PBs came with a separate firmware updater. I did a Google search this morning and got zero hits regarding the existence of a separate firmware updater for the new El Capitan GM installer. Is this just a case of mass oversight on the part of everyone who has posted on the internet so far, or has Apple now integrated the firmware updater into the El Capitan GM installer? Or is such integration even possible? If the firmware updater has been integrated, then that means my friend can no longer download and run it separately (which, for the record, he wouldn't want to do, and I wouldn't want him to do it); therefore, it seems even less likely that he will be able to successfully boot his iMac from the El Capitan OS that I plan to install on his new external HD. What do you think? Technical Details I would probably use my Late 2009 27" iMac (11,1) or maybe another friend's Mid 2010 or Early 2011 MacBook Pro (7,1 or 8,1) to install El Capitan on the external HD. My faraway friend who would be using it with his Mid 2010 iMac (11,2). Thanks.