Steven Seiden wrote in with a situation he’s having with his Mac:
My Mac hangs at a black screen for almost 45 seconds after producing the startup chime before showing the Apple logo. It’s really annoying! Is there any way to fix this?
I’ve seen this before with faulty RAM, as a startup test doesn’t fail, but it generates errors that aren’t severe enough to halt launch. (You can consult our 2014 rundown on troubleshooting RAM problems.)
However, in subsequent email, Steven said he has an 2013 MacBook Pro with 8GB of RAM, and that model cannot be upgraded by a user (or even Apple). After additional testing, Steven discovered that the Startup Disk had somehow become deselected. As a result, his Mac didn’t boot off its internal drive until some time had passed—also a common problem.
Read more here:: How to fix a long pause with a black startup screen in OS X