Getting a “couldn’t unmount disk” error
Trying to erase disk to wipe computer before sale and getting error “couldn’t unmount disk” when I try. Late 2011 MacBook Pro running Sierra.
MacBook Pro 13″, macOS 10.12
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You cannot erase the same drive you are booted from. You need to boot into Recovery Mode, Internet Recovery Mode, or boot from a bootable macOS USB installer. Once you are booted from one of these media you can use Disk Utility to erase the whole physical drive as GUID partition and MacOS Extended (Journaled).
Here is an Apple article with instructions on how to prepare a Mac for sale or recycling:
https://support.apple.com/en-us/HT201065
Note: This article assumes the user is using an SSD with TRIM enabled or a drive with Filevault enabled. If your laptop uses a hard drive or a third party SSD without TRIM enabled, then I suggest you first enable Filevault encryption on the drive and let the encryption process finish before you erase the drive. This way your data will be securely erased since the encryption key will be destroyed when Disk Utility "erases" the drive (it is not really destroying data except it will destroy the encryption key). If the laptop is using an Apple SSD or a third party SSD with TRIM enabled, then you can just use Disk Utility to erase the drive since this triggers the SSD with TRIM enabled to zero out the SSD making data recovery impossible.