Sudo find / -name BaseSystem.dmg -type f 2>/dev/null Open the application 'Terminal', and run the following command: In case Time Machine is not the only culprit, I recommend searching your system for the source(s) of the file. It probably self-deletes this file if there are no issues at the conclusion of the backup. It is very likely that Time Machine creates this file so that it can recover the OS in the case that the backup fails and screws up your system.
For me, it happens right when a Time Machine backup finishes (I am also backing up to an external USB HD).
I have also seen this happen on my system a few times.
