As you know, Google Drive is one of the big players in cloud storage these days. Many also would wonder how secure Google Drive is.
Logically, anything accessible through the internet is not secure. Attacker would find every possible way to hack into it. Though I read somewhere claiming that Google Drive uses encryption of 4096 bit blowfish encoding algorithm, with 7 firewall build by NASA and raid 10 with satellite base backup, one should still have their own physical backup in case of anything go wrong.
In my case, I have my Google Drive folder stored on at least 3 locations – on a server, a laptop and an external hard disk. Better be safe guys.