Due to an unknown error, I was unable to update my 2015 13" MBP from Yosemite to El Capitan. I eventually had to do a clean install of El Capitan, reinstall and re-register all of my software, and copy my data onto the new system (yeah, fun). Before I did that, I copied the entire contents of my Yosemite MBP's SSD to an external hard drive, which isn't a bootable drive. The only thing I neglected to do was export the contents of my Apple Mail mailboxes. My new Mail app re-populated from my iCloud and Google Mail accounts, but I had quite a few mailboxes I'd created that apparently were local and not in iCloud. I have a Mac mini serving as a media center that still has Yosemite on it, but it doesn't have those mailboxes, either. I have two questions: 1) With my Yosemite backup drive connected to my Yosemite Mac mini, how can I unhide the Library folder under my username on the backup drive? Holding down the Option key while accessing the Go menu unhides the Library file only on the mini's system drive. 2) Assuming #1 is possible, is it possible to recover my old mailboxes? What I'm thinking is temporarily renaming the mini's mailbox files, copying my old mailbox files into the mini's Library folder, exporting the mailboxes, copying them to my El Capitan MBP, and importing the exported mailboxes into Mail on my MBP. Or is there an easier way? And if this is possible, what files do I need to make sure to copy?