That sounds like very strange behavior.
If the problem continues, you can investigate using some sort of monitoring software. Maybe start with Microsoft's Process Monitor:
Thanks, yeah I ended up getting a private message suggesting something similar and ended up using process monitor to watch Write Bytes. The culprit ended up being Kaspersky Internet Security which no matter how much I tweaked Windows 7 and Kaspersky to put stuff on the HDD through the registry and symbolic links it still ended up doing this. Must be something sandbox related, but I was forced to remove it and replace it with Microsoft Security Essentials which has solved the problem.
Its amazing how an automated overnight virus scan led to nearly a TB of writes on the drive, that sort of behavior is not cool :/. Well at least now heres a heads up to anyone using Kaspersky, be wary of how much it is writing to your SSD when it scans large files.
The only thing I can think of is because you have an absolute truckload of data on a non-ssd drive, either compressed or in containers. During the scan, it is unpacking to temp folders (SSD) causing the writes.
Can you please elaborate on which version of Kaspersky (right down to the build # if possible) that you were using? was it a recent KIS 2010 (220.127.116.116) or KIS 2011 (18.104.22.1680)?
I just ran a quick test by scanning my (paltry) 290GB of data on a mechanical drive and it has gone from 1.09 to 1.11TB - so that's ~20GB of data. This to me is perfectly normal in terms of the operation of an antivirus scanning packed data but i can see this easily becoming an issue if you have lots of stuff!
It is 22.214.171.1240 and as far as I know there are no large amounts of compressed data on the HDDs. Just about 800 GB of my music, television, and ripped roms since alot of my older games are in storage. Its also no coincidence that the amount of writes added was pretty close to equal to the amount of data scanned that night. I did not realize it ran an auto-scan at first since I usually have them disabled but I had not yet configured Kaspersky since setting everything up on the SSD.
I also noticed similar behavior when downloading large files in chrome (1gb + exes) resulting in Kaspersky adding the same amount of data in Host Writes as the file finished and it scanned. I even tried reinstalling Kaspersky both on the SSD and on the HDD and redirecting my ProgramData folder using a Symbolic Link just in case it was writing temp data there.... but the problem persisted. I'm clueless too as to why, I thought it might be doing some of it's scanning in a protected location and it seems like that might not be it. It was definitely the problem I was having though.