Diskutil

 diskutil list
 diskutil unmountDisk /dev/disk2

diskutil list
/dev/disk0 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         251.0 GB   disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         180.0 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +180.0 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            167.4 GB   disk1s1
   2:                APFS Volume Preboot                 75.9 MB    disk1s2
   3:                APFS Volume Recovery                1.0 GB     disk1s3
   4:                APFS Volume VM                      3.2 GB     disk1s4

With external sd card

diskutil list
/dev/disk0 (internal):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                         251.0 GB   disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         180.0 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +180.0 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            148.1 GB   disk1s1
   2:                APFS Volume Preboot                 75.9 MB    disk1s2
   3:                APFS Volume Recovery                1.0 GB     disk1s3
   4:                APFS Volume VM                      10.7 GB    disk1s4

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *15.9 GB    disk2
   1:             Windows_FAT_32                         15.9 GB    disk2s1

After formatting disk with Disk Utility app.

... 

dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *15.9 GB    disk2
   1:                 DOS_FAT_32                          15.9 GB    disk2s1

diskutil unmountDisk /dev/disk2
Unmount of all volumes on disk2 was successful

Related commands

• mount

See also

• MMC, SD card
• macOS: macOS versions, iTerm2, Homebrew, FileVault, System Integrity Protection, dscacheutil, networksetup, XNU kernel, APFS, diskutil, mdfind, Keychain Access, Growl, Activity Monitor, sidecar, macOS FHS, Apple Notification Center, sw_vers, Spotlight, pbcopy, MacOS application firewall, Applesimutils, Automatic login, security (command), codesign, spctl, Alfred, Xcode, system_profiler, log show, MacOS Preview, Rectangle app, Finder, macOS: Shortcuts, SEP, osascript, macOS profile, UTM, softwareupdate, systemsetup, pmset , macOS tips, macOS Dock, Automator, SIP, System Preferences, installer, cryptex, AirPlay, macOs first setup, Universal Control, menu bar, IPSW

https://stackoverflow.com/a/50933680/14175738