[nhawk]

First if everything was running without a problem when you first installed 4.2 and you've kept up to date with the updates, you can eliminate a problem there right away. But to be sure, I'd disable all mods/add-ons to see if the load drops. If it does, turn them back on one by one to see if the load goes back up. If it does, you found the culprit.

Second, check your site logs for IP addresses banging away at vB trying to hack their way into getting an account. It will be obvious which ones they are because the log entries will all have 'do=addmember' in them and there will be a ton of them all in a row. If there are a slew of them, and they are continuous you need to block that IP from accessing your site.

Third, server load can be affected by so many other things besides vB that many times it takes some detective work to find the problem.

Some examples are continuous email hack attempts, DOS attacks, SSH hack attempts, FTP hack attempts, disk swaps, raid errors, bad memory chips and a bunch of other things. All of which require looking at the server logs to find what is causing the load. And all of which could cause high disk write rates on a server when they write to the logs.