You can easily toggle hidden files by hitting Command+Shift+Period in any dialog window.

"Save As..." used to be a default option in the File menu but now it's hidden by default. Not to worry, with a simple keyboard app shortcut you can regain the uber convenient "Save As..." option in the File menu.

• Open the  Apple menu and choose "System Preferences"
• Select "Keyboard" and then choose the "Shortcuts" tab
• Select "App Shortcuts" then press the + plus button to create a new shortcut for All Applications
• Set the following for the keyboard shortcut:
Application: All Applications
Menu Title: "Save As…"
Keyboard Shortcut: click into the field, then press Command + Shift + S
• Click "Add" to finish adding the "Save As..." keyboard shortcut and close out of System Preferences

The "Save As..." will now appear by default in the File menu of apps, and be available instantly as the Command + Shift + S keyboard shortcut. You can test this out yourself by going to any app that supports file saving and you'll find the File menu now includes the "Save As..." option by default, along with the keyboard shortcut.

The modern macOS default keyboard shortcut for "Save As..." is the finger twisting combo of Command + Option + Shift + S. All we're doing in this particular App Shortcut is to remap that complex keystroke into the familiar and easier to manage Command + Shift + S keystroke, which was the default in Mac OS for much of Mac history. This change may not be for everyone, but if you're a fan of using "Save As...", you'll undoubtedly appreciate knowing that you can get this great file saving feature back with a simple effort.

• Double-click a file's icon open it
• Select a file and press Command+C to copy a file's URL
• Select a downloaded file and drag it to a folder, to your Desktop, or even to a Dock icon to launch it with that application

Apple’s Mail and most email clients on every platform and from every third party optimize their defaults settings around leaving mail on a server. That’s the modern way, where we can reach the same mail storage on any device, as well as through web mail.

However, it’s possible to avoid all this. You just have to change a view settings and rethink how you file mail once you’ve dealt with it. You can archive messages on a single Mac and store them there without leaving a copy on the server but you’d better be making backups, Time Machine or otherwise, or you’ll be sunk if your drive fails.

Your Inbox will always remain on the server. The Inbox is essentially a window into messages that have arrived and you haven’t processed. You make one change in settings and one change in behavior for everything else.

In Mail in macOS, follow these steps:

• Select Mail > Preferences and click Accounts.
• Select iCloud in the list at left.
• In the Account Information tab, set Download Attachments to All (the only reason to avoid this if you get frequent large attachments in messages you delete without needing those attachments).
• In the Mailbox Behaviors tab, for each mailbox popup menu, select a mailbox listed under On My Mac (if no mailbox exists, you need to create it from the Mailboxes sidebar).

Now all the standard Mail behavior will result in messages being downloaded or stored on your Mac.

On the behavior side, create all the mailboxes you need under the On My Mac section of the Mailboxes list, and then as you receive email in your Inbox, file those locally into those folders. That removes the messages from the mail server, leaving it stored only on your Mac.

HDDs

Unless you’re dealing with secrets that would lead to the overthrow of governments, using Disk Utility’s secure erase feature meets the mark. HDDs can also be physically destroyed with a drill equipped with a bit suitable for puncturing the metal casing. A hammer and chisel could work, too. If you have a dead HDD and if you think anyone with motivation might pay to have the data recovered, physical destruction is the only way to ensure data isn’t readable.

SSDs

Data is written in an unpredictable fashion on SSDs to distribute the wear across all the memory cells in the solid-state device. As a result, a secure erase feature doesn’t work at all, as it may not overwrite all the data. Physical destruction is really the only course of action, which is an unfortunate waste of technology. And if you have a Mac in which the SSD isn’t removable, but part of the computer, that’s even worse.

RAM

Fortunately, the various kinds of RAMs used by generations of Macs are all volatile memory: the contents disappear instantly or shortly after a device is powered down. So far, there’s no way to recover any traces of data from RAM chips.