Such is the life of a writer,sending off the most personal thoughts possible to his hard drive.
It's the hard disk drive in your computer, where you store files you've created. When you save a file, you're typically saving it to your hard drive.
Although we'd usually use a verb like saving or writing, the author here has chosen sending. This is a more figurative way of expressing the same thing, which is appropriate with such poetic-sounding expressions as "Such is the life of a writer".
Here, the writer is writing "the most personal thoughts imaginable" in a word processor. The writer then saves these thoughts to disk. This is a fact of life for a writer, according to the sentence you've quoted.