When I transfer to an exteranl drive I always finish the transfers,then check that everything is on the exteranl drive and the delete from the hard drive.
We all get tired and ratty on these long boring tasks,some advice is if you have been at a task a long time and getting tired - leave it - there is always another day.
Sorry to hear of your horrors but these events sometimes make the rest of us use the grey matter and think about what we are doing.
Sooooooooooooooo sooooooooooorrrrrrrrrryyy to hear of your disaster we all know how you must be feeling.