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This question already has an answer here:

I working on macOS Mojave 10.14.5

I am new to macOS and I don't know much about installing different versions. I can't dare to uninstall the current as I am afraid to lose my file and installed packages.

how can I safely downgrade to 10.11 or 10.10?

Updates: info about my MacBook:

MacBook Pro (Retina, 13-inch, Late 2013)

The reason why I want this is that I want to install wifi USB adapter but it seems that it's only compatible with os 10.11 or below that. Having this wireless adapter working is so much important to me though.

Adapter details: EDUP 802.11n USB Wireless Model : EP-N8553

marked as duplicate by Graham Miln, Nimesh Neema, nohillside macos Jul 15 at 19:23

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    How old is the Mac? You cannot downgrade to any OS older than the Mac itself. In order to obtain any older OS at all, you need it to be in your purchase history & in fact you need to already be on High Sierra or earlier for even that to work. See apple.stackexchange.com/questions/309399/… – Tetsujin Jul 15 at 15:43
  • I added more details. check them out above – Cs20 Jul 15 at 15:46
  • OK, so it would theoretically run them... if you can get hold of them. – Tetsujin Jul 15 at 15:48
  • how can I do it ? – Cs20 Jul 15 at 15:50
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    What is wrong with the Mac's Wifi? The adaptor you've listed seems to be very cheap and old: wouldn't it be better to buy a newer, better one that works with your Mac without drivers? Moving backwards to 10.11 will cause all sorts of compatibility problems. Also: you say you are afraid to lose your files. Please tell me you have a backup? – benwiggy Jul 15 at 16:39
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There is no downgrade option available which let's you 'go back to a previous macOS version'. Most Mac Apps automatically convert your Photo, Mail or iTunes files to a newer database version. So you won't be able to import Mails out-of-the-box from Mail (Mojave) into Mail (El Capitan).

  1. You will need to backup your data, first
  2. Erase your disk or volume using Disk Utility
  3. Reinstall from macOS Recovery or USB drive.
  4. Move back your data from backup

There's no need to reinstall your system due to USB adapter issues. It rather sounds like a incompatilbity issue. Your USB adapter is too old. Try using your USB adapter on another friends, collegues computer for further testing.

The only guaranteed step is to erase the system, reinstall a clean OS and only migrate / import data that was backed up to the lower version of the OS or older. You generally can load an older backup on a newer system, you generally cannot load a newer backup level on an older system.

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    Note that even after backup, you may not be able to restore data to an older OS. It may not be compatible. Apple Mail is a good test of that one. – Tetsujin Jul 15 at 17:09
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    @Tetsujin yes, good point. I have updated my answer. Thanks. – howdytom Jul 15 at 17:22

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