Update to Sierra on External ssd without dataloss.

[Joydexon]

Hello everyone, I had a bad issue with logic board of MacBook Pro 2013, the only solution has been to remove the internal ssd card and plug it into a usb3 reader in order to boot it on an iMac I have, and continue to work on important deadlines, but right now the only problem (I assume) is that the iMac runs Sierra 10.12.6 and the external ssd has Mavericks 10.9.5 on it, so the iMac won’t recognize it as bootable, showing the Prohibition Symbol when I tried to start from it. The disk utility infos say bootable : yes.

Is it possible to update the OS of the external drive via Recovery mode without data loss (got a backup already for the worst scenario)? Are there other solutions? Thank you very much.

 

[Fishrrman]

You may have found the solution already, but...
If you have the apps you need on the iMac, there's no need to "boot from" the SSD.

Just copy the data/documents you need to work on from the SSD to the iMac's drive and work on them that way.

Something you may need to do to overcome "permissions problems":
1. Connect the external SSD and let its icon mount on the desktop
2. Click on it ONE time to select it, then...
3. Type "command-i" (eye) to bring up the get info box
4. In get info, at the bottom, click the lock icon and enter your password
5. Put a checkmark into sharing and permissions "ignore ownership on this volume"
6. Close get info.

You can now copy anything on the SSD to the iMac, and it will "come under the ownership" of your account on the iMac.