Where can I find the log of all mounted volumes?
You can find the information in /var/log/system.log (or using the Console application). But the information is not kept forever.

For example:

Aug  4 07:34:34 ******** kernel[0]: hfs: mounted External on device disk3
Aug  4 07:34:44 ******** kernel[0]: hfs: mounted Time Machine on device disk5


and and a DMG

Aug  5 07:53:25 ******** kernel[0]: hfs: mounted Xcode on device disk6s2


After a while system.log is compressed and archived. To search in all the stored logs

cd /var/log/
(sudo cat system.log; sudo gzcat system.log.*) | grep mounted