[y2ksw]

I'd say the disk is a little too small in order to have rapid access for 18 million posts + attachments. I estimate the database size about 130 GB, which is a very optimistic size (likely to be more), and about 50 GB of attachments. This would make a 300GB disk at 2/3 full and thus becomes already unefficient.

The suggested RAID configuration would give 2x5 disks of which 1x5 disks are serial and the other 1x5 array the backup (parallel). At 1.5 TB (at 300 GB each) the used space is about 2/15 or 14% which is just fine.

It may also work with a single drive, but please make sure to have daily or progressive backups. A disk failure may become a major issue if backups miss or lack quality, and data recovery from broken disks is very, very expensive ($10k+).