Question about booting from external drive

[Loke2112]

I have been happy with Mojave on my 2012 iMac but am curious if I can try Catalina again for a number of reasons by booting from an external SSD. I have everything set up from Apples own instructions but am not sure what to do when the 4 options appear. I was told that I can easily migrate data and apps over and the rest via iCloud. I have a time machine back up on another external drive of my Mojave set up. It's just not clear what to choose. Restore from Time Machine seems to be the only option yet that sounds like it will just instal my current state of Mojave.

Worst case scenario I wouldn't mind just moving my current state of Mojave to the SSD and freeing up the internal drive.

Can anyone advise? TYVM in advance if so.

 

[Fishrrman]

You can set up and boot from an external drive just like you would do so with an internal drive.

I would suggest that if you're going to try Catalina on an external drive, that it be an SSD (instead of a platter-based hard drive).
It will run much MUCH better.

Might be worth buying a "cheap, off-brand" small-sized SSD for such an experimental task.
You can get 120gb SSD's for less than $20.
Put it into a case like this:

SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)

SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)

tinyurl.com

Connect it to your iMac.
Do the OS install.
When done, you'll then be "booted to" the external, so...
Begin the setup process.
When setup assistant asks if you wish to migrate from another Mac or drive, YES, do this.
Just point setup assistant at your internal drive and bring over what you want. Being a small drive, you probably won't be able to "bring it all" over. Just what you need.

Now you can "switch boot" back-and-forth as desired.

 

[coldshot]

Hi, thanks for the posting this thread. I would like to do the same to speed up my 2017 iMac. I have a 500 GB T7 SSD and want to use it as much as possible, though it's not big enough to move everything. If I understand correctly, install Catalina OS on SSD, and then use it to boot from. My question is which applications should I move over to the SSD if I can' t move it all? I am tired of my iMac's slow response to basic tasks. Any advice? I'd rather not buy a 1TB SSD if I can avoid it, but I am a complete novice and want to optimize what I already have. Thanks in advance.

 

[Erehy Dobon]

I suggest acquiring an appropriate sized external SSD, making a bootable clone of your current macOS system drive, booting off the external clone, then upgrading that to Catalina/Big Sur.

This allows you to test the functionality of many of the apps you currently use. If you install Catalina/Big Sur on a blank drive, that's probably not the work environment that you currently rely upon.

In my case, I have both. I have a moldy old 120GB SSD (circa 2013) that is large enough for blank macOS installations but not large enough for a clone of my normal boot drive. I have a 500GB external m.2 SSD that is. When I seriously consider upgrading my Mojave Mac mini 2018 to Big Sur, I will make a full bootable clone of my Mojave system drive to the external m.2 drive and keep that on Mojave until I get rid of the Mac mini 2018.

 

[Fishrrman]

coldshot wrote:
"My question is which applications should I move over to the SSD if I can' t move it all?"

How large is the INTERNAL drive in the iMac?

The external boot SSD should contain:
- The OS (of course!)
- ALL your applications
- Your accounts (slimmed down)

By "slimmed down" accounts, I mean that you should leave the "large libraries" of movies, music and pictures on the slower HDD if they won't fit onto the SSD.

Leaving them on the HDD won't hurt anything -- with apps such as iTunes, Music or Photos you just "redirect" the apps to use the libraries on the internal drive (rather than on the SSD).

The idea is to keep the SSD "lean and clean" so it will run at maximum performance.

 

[coldshot]

I suggest acquiring an appropriate sized external SSD, making a bootable clone of your current macOS system drive, booting off the external clone, then upgrading that to Catalina/Big Sur.

This allows you to test the functionality of many of the apps you currently use. If you install Catalina/Big Sur on a blank drive, that's probably not the work environment that you currently rely upon.

In my case, I have both. I have a moldy old 120GB SSD (circa 2013) that is large enough for blank macOS installations but not large enough for a clone of my normal boot drive. I have a 500GB external m.2 SSD that is. When I seriously consider upgrading my Mojave Mac mini 2018 to Big Sur, I will make a full bootable clone of my Mojave system drive to the external m.2 drive and keep that on Mojave until I get rid of the Mac mini 2018.

Thanks for the response! Yes...I thought a bootable clone would fit on my 500GB, and went through the process but it was just over. I do have a 1TB in my sites at the moment which would solve all issues.

 

[coldshot]

coldshot wrote:
"My question is which applications should I move over to the SSD if I can' t move it all?"

How large is the INTERNAL drive in the iMac?

The external boot SSD should contain:
- The OS (of course!)
- ALL your applications
- Your accounts (slimmed down)

By "slimmed down" accounts, I mean that you should leave the "large libraries" of movies, music and pictures on the slower HDD if they won't fit onto the SSD.

Leaving them on the HDD won't hurt anything -- with apps such as iTunes, Music or Photos you just "redirect" the apps to use the libraries on the internal drive (rather than on the SSD).

The idea is to keep the SSD "lean and clean" so it will run at maximum performance.

Thanks so much for your response. My internal drive is 1 TB and is currently at 498.21 GB. My Carbon Copy Cloner couldn't finish the cloning. I "just" purchased the SSD two months ago BEFORE I found this forum topic and regret not getting the 1 TB, but I still want to get value from my SSD. Redirecting sounds exactly the way to go as I still want to use both drives. Photos staying on the HDD would likely solve the storage problems. Not sure how to redirect but look forward to it! Do I still use CCC for all this? My SSD is formatted correctly and ready to go as per a previous post/thread.