How can I find out what is reading and writing so much data on my MacBook? In the 20 or so days that my retina MacBook has been turned on, in Activity Monitor I can see that around 2 TB of data has been written and 1.2 TB read. Most of the written data seem to come from kernel_task, with about 800 GB written to disk. However, altogether the processes in Activity Monitor add up to only 1 TB written (not 2 TB, which is the total data written). What's even stranger is that the sum of the read data adds up to about 250 GB, which is much lower than the 1.2 TB read that Activity Monitor shows. When observing Activity Monitor, the data read or written per second goes to around 100 MB/sec even 15 seconds or so.

How exactly can I figure out what is eating my disk like this? I'm afraid that if this continues, my total read/write amounts will go up to nearly 40 TB in a year, and eventually wear out my SSD more quickly than it should.
This happened to me once. I used Daisy Disk to figure out where the disk usage was coming from.

In my case, there was a problem with the coresymbolicationd cache, which was growing indefinitely, and continuing through reboots and reinstalls of the latest Combo Update.

After deleting /System/Library/Caches/com.apple.coresymbolicationd, everything returned to normal.

Next time this happens, I recommend taking a look with that utility.