[com.apple.TimeMachine:TMLogInfo] Ejected disk3 from '/Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D/Graham's_Mac.sparsebundle'
[com.apple.TimeMachine:TMLogInfo] Failed to unmount '/Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D', error: 16 Resource busy
[com.apple.TimeMachine:TMLogError] Failed to remove directory '/Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D', error: 16 Resource busy

Grahams-Mac:Volumes Graham$ sudo lsof /Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D
COMMAND PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
mds      82 root   15r   DIR  52,32      264    2 /Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D

Grahams-Mac:Volumes Graham$ sudo mdutil -i off /Volumes/com.apple.TimeMachine.TM_Prime*
/Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D:
2019-04-27 18:22:26.919 mdutil[2929:1075028] mdutil disabling Spotlight: /Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D -> kMDConfigSearchLevelFSSearchOnly
    Indexing and searching disabled.

Grahams-Mac:Volumes Graham$ diskutil umount force /Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D
Unmount successful for /Volumes/com.apple.TimeMachine.TM_Prime-DA16D91F-3320-4834-91ED-2902CE57E46D