Open Console in Utilities & see if there are any clues or repeating messages when this happens.
See if the Disk is issuing any S.M.A.R.T errors in Disk Utility...
Looking at activity monitor was in fact the first thing I did - but while there's a lot of disk activity, there seems to be very litte CPU involvement.
The kernel_task is the most activite one (beside activity montor task of course) but mostly in the range between 0,3 and 1,5%. Other active processes are almost always below 1%. I'm not sure what to comsider as "too much" in this context?
Open Terminal in Utilities and enter
Hit return, after which you will be prompted for your admin password. You won't see it as you type it in. And you will be given a warning, then hit return again.
You will have to scroll down -- use the down arrow -- looking for all the writes and which process is making them. There will be a lot to look through, but this may give you a clue.
To quit, enter Control-C.
Just to be sure, did you make the change in Activity Monitor that BDAqua suggested? Makes a huge difference as, by default, AM shows only a limited number of processes.
Any third-party "pay for" anti-virus or security software running?
Perchance do you have the game "Prey"? Its demo caused my MacBook Pro to continuously write to system.log, ballooning that file from normal 200-300k to over 65MB. The game did not have to be running for it to act out.
Hi WZZZ and BDAqua,
My HDD have stayed rather calm for a number of days, but is now chugging along again. I did run the sudo fs_usage and these are the processes I see (the two first are the most frequent):
For Allan Jones: Yes- I'm viewing all processes in activity monitor.
Searching, hamachid appears to be related to Logmein. Do you have that?
mdworker and mds are both Spotlight. For that, try reindexing Spotlight.