I’m switching to English as the main language for this blog, one reason is try to improve my English writing, so please excuse my poor English.
This story begins when I was in visiting US. I just take my laptop and one external HDD with me, so all my photos taken in US are stored in my external HDD without any backup. Once, I managed installing win 7 on my laptop, I accidentally format one of my drive, and failed to recover them. (The detail story is I changed the partition scheme which change the partition table, then a lot of copy/write to the HDD, so it is impossible to recover the data) That accident cost me about over 100 photos which I taken at downtown seattle, from then on, I became extremely serious about backup.
So what to backup. Generally, your life data is the most important and should be first priority for backup. Almost all other data can be retrieve via different ways. E.g. you can buy the music/movie you lost from store, softwares maybe more expensive, but you still can make it. But for the data about you or created by you, most likely you can not retrieve them via commercial purchase.
Back up all the materials created by you, your documents, diary, photo, family video. maybe more like your game score, email archieve etc.
Backup now, because there are some data in your computer is the only copy of that digitalized information in the world.