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macrumors member
Original poster
Jul 10, 2018
Trying to install Mojave in the best way on an external SSD to use as a system for my little older Imac.

What are the best settings in formatting the drive to do this?

Also, will the drive lose speed if I encrypt it?


macrumors Penryn
Feb 20, 2009
"What are the best settings in formatting the drive to do this?"

Are you going to do a completely fresh install using the installer app?
Then, format the external drive to "Mac OS extended with journaling enabled, GUID partition format".
It will get converted to APFS during the install.

"Also, will the drive lose speed if I encrypt it?"

I don't encrypt my drives, can't help with that.

An alternate course of action:
IF you already have "a good copy" of Mojave up-and-running on another Mac, you could use a cloning app (CarbonCopyCloner or SuperDuper) to "clone over" the OS to the SSD.
By using CCC's ability to "selectively clone" (leave some files behind), you could create a trimmed-down working copy of the OS with ease.


macrumors regular
Mar 19, 2019
Southern NH
I encrypt my MBP as it travels with me on occasion. I don't notice any real speed differences with my 2012 fitted with an internal SSD and using 16GB of RAM.
If that was my older iMac, I'd do the internal disk swap with a good sized SSD, 1TB or more. You can also add an SSD, but it is a much more complicated procedure, and requires a kit if your iMac didn't come with internal SSD. Check out the video tutorials at OWC. I've never been inside an iMac, but almost every other Mac. The only way to boot externally without severe loss of speed is Thunderbolt, which requires expensive cables and enclosures. With USB3, you go to 1/2 speed of the SSD, and with USB2 or FW, forget about it. That depends on the model year. TB started with the 2011 models and USB3 began with 2012. So pre 2011 models are a no brainer for internal drive replacement. This will give new life to an old iMac.
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