OS on external SSD, leave data on spinning drive?

iMac / Late 2012

Anyone have experience with installing High Sierra on an external SSD (USB 3) and leaving all User folders/data on the spinning drive?

Specifically - how do I point my formatted SSD to use the User folders on the internal HDD?

Current Setup:
iMac HDD

  • High Sierra OS
  • all applications
  • User accounts and associated data

Desired Setup
External USB 3.0 SSD

  • High Sierra OS
  • all applications
  • [user accounts pointing to iMac HDD]

iMac HDD

  • User accounts and associated data

Never tried it myself, but found this.

1 Like

I used to run my Mac Pro with the OS on an SSD and my user home directories on the original spinning drive. At a high level I:

  • Backed up the old drive to a clone backup
  • Installed the OS on the SSD
  • Created a local admin account on the SSD (in case the second drive ever becomes unavailable)
  • Reformatted the spinning drive
  • Mounted the spinning drive as something other than the default Users location (e.g. /export/home)
  • Created a my regular user account with a home directory pointing to the location of the spinning drive mount point (e.g. /export/home/user1) making sure to keep the userid and groupid the same as the original account
  • Copied the user account home directories from the backup clone to the internal drive
1 Like

Thank you, JohnAtl.

I tried this with one of our smaller user accounts (i.e., one of our kid’s accounts…). When I logged in as her after the fact, I saw numerous errors about having to repair the keychain, and some others which I don’t remember. MacOS asked me if I wanted to either Repair or Reset (can’t remember) some things so I got a little nervous.

Ultimately I backed-out by booting off the spinning drive and logging in as her again.

Maybe I did something wrong…

I converted a iMac with spinning drive and optical drive to SSD & internal much larger spinning HD. Actually I should say I had a local shop do the hardware work. But it involved a new clean installation of the OS on the SSD & moving all the data back on. Under those circumstances and following the same directions linked above, it works great.

Shouldn’t be a problem. In 2007 I bought iMac which I used around the house for 7 years, and 5 years in the internal drive got fried. I just switched to my bootable clone backup as the main drive and it lasted the rest of its lifetime. Back then, I was connected via a Firewire 400 or 800 port, so access/read times were definitely felt. But today, with faster modern connectivity and SSDs I imagine it would be almost indistinguishable from having an internal SSD.

My setup -

iMac / Late 2013

External Thunderbolt 256 GB SSD - 70 GB used

High Sierra OS
User Accounts
Documents - current documents and scans

iMac 1 TB HDD - 660 GB used

Photos libraries
Archived documents, etc

External SSD is so much faster than using the internal HDD for the system. I decided to put the user folder on the external SSD so that I could clone an entire system (including user accounts) with CCC. Thought it would be easier to restore should something happen. Everything is backed up with Time Machine as well as iDrive (online backup).

1 Like


So your larger files are on the HDD (e.g., photos, etc.). This setup might help me.

I purposely bought a smaller (128GB) SSD for cost savings while my HDD is 1TB.

To mimic what you did, it seems I would need to:

  1. Use Migration Assistant to move the smaller User folders to the SSD. (Assuming Mig Asst allows you to pick only certain users…)
  2. Create a new user profile on the SSD for me (all Photos are on my account, most of the data, etc), and leave the data on the HDD.
  3. This is where it gets challenging for me: how to I point my account to the HDD for data without receiving all the errors about Keychain etc.?

Here are my notes on part of the process I followed:


Got new 250 GB SSD and 2 TB internal HD to upgrade 2010 iMac
Fantastic decision for $500 incl. labor and external housing for my old HD, it’s like I have a brand new computer (except of course no Retina display, handoff)
Challenge was how to migrate iTunes bc not enough space on the SSD for music
Also concerned bc I kept my Apple Music subscription after the trial incl. iCloud music library and was concerned that if I ended up with a situation where iTunes couldn’t find files, it would start replacing them with Apple Music equivalents (which don’t always match & have other reported issues as you know…). I also wanted to end up with the iTunes database (but not the media) on the SSD. This was all potentially complicated by the fact that I hadn’t backed up iTunes in the way Apple advises, or even used the default location (Music/iTunes/iTunes Music); instead I had it all self-organized in folders by letter – but not consistently so j-k, u-v and some others were in combined folders…).
No guidance online for this triply weird scenario
Could experiment since I had a complete backup which wouldn’t be changed.

Here’s what I did:

  1. Copied contents of old HD Music folder to a folder on the new HD (User Files/username/Music) then ejected the old HD
  2. Turned off my wifi so I had no active internet connection (hence no connection to Apple Music)
  3. Created a symlink from Users/username to User Files/username/Music on the new HD
  4. Took a deep breath and started iTunes
  5. Everything worked as expected - iTunes Store warned me it couldn’t connect, Apple Music playlists were greyed out, but all local music and playlists were working fine.
  6. Turned on wifi, put in my Apple ID password a couple times, updated payment info, and all looked well except Apple Music playlist contents were still grey
  7. Restarted iTunes - Apple Music came back

At this point everything looked good except the iTunes Library was on the HD not the SSD.

  1. Copied the updated iTunes database (whole folder XXX) to Users/username, and to a temporary backup location in case the next step failed.
  2. Deleted the iTunes database at User Files/username/Music/xxxxx (on the HD) that I had just copied
  3. Started iTunes again
  4. Changed the library location in prefs>Advanced to User Files/username/Music
  5. Everything worked but I realized that when I moved the library to the SSD I also moved 40 GB of iOS app backups
  6. Consolidated files, checked by reveal in finder that apps in iTunes in fact were using newly consolidated files on HD, and deleted old iOS backups
  7. Moved cruft from old manual organization to a Music archive directory (which also had stuff I’d previously removed from iTunes in order to get below the 25k song limit)
  8. Only problem left was that consolidation had also created about 50 gb of duplicate movies
  9. Used Macpaw Gemini to delete duplicate originals

So at the end of the day
Blazing fast library on SSD with media on HD
Clean organization and purge of dupes

Not sure if this is still pertinent but here is my process.

1 - I have multiple backups, Time Machine and iDrive (cloud backup service), so if anything went wrong I wouldn’t lose anything.

2 - I copied everything I wanted from my iMac internal HDD to an external HDD - Documents, Photos library, Music folder, etc. I made sure everything was there. Re: Apple Photos —- I keep my full library on my Mac (full size versions), not the optimized library.

3 - I installed High Sierra on the external SSD. During installation I logged into my Apple account to restore my settings etc. I did not use the Migration Assistant. Note: I did not allow iCloud to sync my photos at this time because I did not want 120 GB of photos downloaded to the SSD.

4 - I started High Sierra from the SDD and made sure it worked properly.

5 - I reformatted the iMac internal HDD. I wanted to start from an empty HDD.

6 - Moved everything from the external HDD (see step 2) to the newly formatted iMac internal HDD.

7 - I opened the Apple Photos library from the iMac internal HDD (open from the library itself, not the Photos app). Went to settings and checked “icloud photo library” and “download originals to this Mac”.

8 - After you are sure Apple Photos works properly and it is using the library on the iMac internal HDD, you can delete the library that Apple made in your Pictures folder on the SSD.

9 - Opened iTunes and indicated which folder on the iMac internal HDD to use.

10 - If you want downloads to go to your iMac HDD, you can indicate a folder in your web browser settings.

11 - At some point, should you need to clear out space on the SSD, you can move more stuff to the iMac HDD.

The main things: 1 - Backup everything before you start. 2 - Be cautious at every step, but if you are not, refer to 1.

Good luck with whichever method you use.

1 Like

@neonate thank you.

So if I understand correctly, on a day-to-day basis your SSD is where your new documents go unless you explicitly put them on the iMac HDD.

Or are you using iCloud Drive?


So if I understand correctly, on a day-to-day basis your SSD is where your new documents go unless you explicitly put them on the iMac HDD. – Yes. My current home and work documents, ScanSnap scans, etc go to folders on SSD. An example would be — scans of all my current bills, etc would go to SSD. At the end of the year I archive these to the iMac HDD.

Or are you using iCloud Drive? — I do use iCloud Drive for a few things, mostly Numbers spreadsheets that I want to access anywhere (iPad, phone, web).

This system seems to work for me at present. I’m always tweaking my workflows. Let us know how it goes for you.

1 Like

Here’s what I ultimately did…

I bought a 1TB SSD to match the size of my spinning drive.

I used Migration Assistant to copy most everything to the SSD, including the data.

So far so good – I’ve only been running from the SSD for less than an hour. it’s significantly faster!

1 Like

Congratulations! One thing to do to keep your SSD running smoothly is to enable TRIM on the external drive, something that has not been enabled by default for Mac drives. (Unless something has changed in the last year.) Read these articles:

There’s also a $15 app called TRIM Enabler, but I’m not sure you need to use it if you follow the instructions in the 1st two articles.

Sounds great. The 1TB SSD is external? What do you use the spinning drive for?

@neonate My 1TB SSD is external on the iMac. I currently am not using the internal 1TB spinning drive.

In a month (i.e., after some runtime…), I’ll likely do a secure wipe of it. My backups are via Time Machine to a Time Capsule.

1 Like