"rik229 Said:
"15.6.1 security update failed: Thanks for your suggestion --"
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You are welcome. Read the following...
What Reinstalling Does:
Q1. "I'm not even sure it's properly installed in my recovery disk. I could end up with Yosemite again!"
A1. Reinstalling does not negatively affect items, nor will it result in a downgrade. Sometimes, update do not go through 100% as intended, and reinstalling a macOS will get things back to how they should be, which is what reinstalling the macOS is all about.
Avoid Scams on New Macs:
Q2. "I'm also leaving in two weeks and will be gone (with my computer shut off) for four months. When I get back I plan to get a new computer"
A2. If you plan to get a used Mac, have the user perform as instructed on this link, including the iCloud logout (so you can use Migration Assistant): What to do Before you Sell, Give Away, or Trade in your Mac - Apple Support. If they do not log out of iCloud, then it will be useless to you.
Why to Install Updates:
Q3. "Is this recent security fix all that important, or can it just wait till I get back? How do the hackers get access to our computer? Are there certain websites to avoid? thanks ;-)"
A3. No one is going to hack your Mac. If anything could be hacked, its an iPhone (because they can be jailbroken when purchased). Note that security updates should be installed, as they contact secure measures (for things like bugs, inoperability, and incompatibility). If you plan to sell it, then let things stand, storing it in a safe location while you are gone. When you get back, use the link I've just provided.