Any disk access to a failing drive can cause a stall or a complete hang. Disk I/O is a "blocking call" in the kernel, regardless of whether it's the boot volume or any other attached storage. So during startup off a good drive, a failing drive that the OS tries to access to query or to mount can cause problems. I've also seen an optical drive do the same thing, even when no disc was present. (I've also seen multiple computers that could not start off any internal or external drive until I unplugged the failed optical drive)
Thanks for the additional input. I still have hopes of recovering the system at least for use as a file/print/Time Machine server. From your description I realize the failure could be either of the internal drives, the SSD or HD.
My first task will be to retrieve the 600 GB of tunes, audiobooks, and photos off of the HD. Thanks to iCloud all of that is recoverable, but it would be a lot faster if I could extract it directly. The same thing is true of the Desktop and Documents folders as well as Keychain, Evernote, Safari, Mail,
etc.. I also have a viable Time Machine backup, but because of everything I have been doing on the machine, I will probably reinstall all of the apps
etc. from scratch — It has been years since I did that so this is a good excuse to get really clean.
