How to find recently connected external disks?
The following commands will show you all the external disks which were mounted upon your Mac during the last 7 days (unless you modified the conservation limit of system logs):

grep mounted /var/log/system.log
zgrep mounted /var/log/system.log.*



If you would like to increase your auditing possibility on this sort of event, here is the way (for the ones at ease with the Unix talk):


open the newsyslog configuration file:

/usr/bin/sudo vi /etc/newsyslog.conf

change the following line:

/var/log/system.log                     640  7     *    @T00  J


with:

/var/log/system.log                     640  30    *    @T00  J


(which means keep 30 versions of my old /var/log/syslog files)
save this modified /etc/newsyslog.conf.