Can you quantify the performance of the 10k sata drives? I've installed mine, and I can hit IO bottlenecks. I only have 8000 accounts, and am running on a single server, with a software raid10 4/7200 rpm WD500's, NCQ. Dual dual-core, each core barely utilized, memory is fine, not running antispam/virus, I note occasional IO bottlenecks. I'm running DRBD to a secondary server as well.
Wondering if moving to 4 Velociraptors would get me by until some of the new drive tech comes along.
The number of accounts is not the point here.
The point is the number of incoming (and outgoing) mails, if indexing is enabled (or not), the number of concurrent users and the way the users connect (ZWC vs pop vs imap).
Comparing the storage usage means comparing all of this...
Are you 100% antivirus & antispam are not running? They are I/O monsters. POP3 is also a hog for disk I/O and as Klug has rightly mentioned - indexing. Remember in my instance we had large, large numbers of disks on a SAN, striping across many.