I erased the internal disk of my MacBook Air (2016). Since there is no OS anymore, I turn it on in the recovery mode (Cmd + R) and want to install the macOS Sierra on a 2 TB external hard drive. In the past, I partitioned such hard drive and allocated 500 Gb to Time Machine backups and the rest for my files. If now I install the macOS, do I need to format it or the OS will be installed on the free space (there is still a lot of space).

Thanks for your help!

  • The macOS installer has a disk check and will let you erase / partition. It’s in the steps for an erase install. Depending on how you get the sierra installer, your steps may change. Do you have that part figured out?
    – bmike
    Feb 22 at 20:13
  • Thanks for the answer @bmike. The installation of Sierra starts directly from the Recovery window, since that’s the only thing I can access due to the fact there’s no OS anymore. I only see the option “Reinstall macOS” and there is just Sierra available.
    – nd091680
    Feb 22 at 21:22

3 Answers 3


You'd be better off formatting the drive prior to installing the OS.

The macOS Installer will allow you to choose from any available and compatible partitions (you may need to click 'Show All Disks').


Adding on to @da4, make sure to partition the disk, not make APFS containers. This will give fixed size boundaries to each, instead of creating a weird scenario where you don't have any space left on your hard drive because TM consumed all of it.

  • Thanks. So this means that using partition is different from the “new” system through APFS? Sorry maybe it’s a stupid question but I really don’t know much about it.
    – nd091680
    Feb 22 at 21:25
  • 1
    @nd091680 They both coexist nicely. The APFS volumes allows different disks to share the same storage without a hard barrier. For example, I have an extra volume on my boot drive that I dedicate to old files. I can let both flexibly share the same storage. However, TM likes to preserve backups & use as much storage as possible. Therefore, you have to limit it with a hard-partition.
    – anonymous
    Feb 22 at 21:54
  • 1
    Everything is more clear now, I think I’m gonna proceed like this. Thank you again for the prompt response.
    – nd091680
    Feb 23 at 9:10
  • 1
    @anonymous What you say is mainly true, but note that for any non Time Machine APFS volume you can add a quota limiting its size
    – mmmmmm
    Feb 23 at 11:49

Here are step by step instructions to use recovery OS to erase any disk attached to your Mac.

The utility menu looks a little different on sierra, but look for utilities in the boot screen menu bar. You do not want APFS for this age mac or operating system. HFS+ journaled is ideal.

  • Fantastic, thank you!
    – nd091680
    Feb 23 at 9:12

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