Error: -69888: Couldn't unmount disk I'm on a Mac OS Catalina trying to wipe a drive via command line. 

⚡️  Desktop  diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     314.6 MB   disk0s1
   2:                 Apple_APFS Container disk1         1.0 TB     disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +1.0 TB     disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD            10.8 GB    disk1s1
   2:                APFS Volume Macintosh HD - Data     843.8 GB   disk1s2
   3:                APFS Volume Preboot                 80.6 MB    disk1s3
   4:                APFS Volume Recovery                526.6 MB   disk1s4
   5:                APFS Volume VM                      4.2 GB     disk1s5

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Lacie 1 TB              999.9 GB   disk2s2

⚡️  Desktop  diskutil eraseDisk ExFAT NONAME /dev/disk2
Started erase on disk2
Unmounting disk                                                                                                       
[   \   \   \   \   \   \   \   \   \   \   \   \   \   \ ]  


I kept getting 

⚡️  Desktop  diskutil eraseDisk ExFAT NONAME /dev/disk2
Started erase on disk2
Unmounting disk                                                                                                       
Error: -69888: Couldn't unmount disk
Run

diskutil eject disk2s2


Repeat that and sudo fuser -c /dev/disk2s2 to kill or quit any processes that are blocking the eject.