The chassis and the power design. The 850 moves to a 2U enclosure with dual, redundant power supplies, something the 650 doesn't have, on top of jumping to 18 TB and around 2,500 mailboxes. RAID-6, ECC memory, and the core archiving features carry over unchanged from the 650.

The archiver keeps running on the second supply without any interruption to archiving or search. You get an alert to replace the failed unit, but there's no downtime and no need for an emergency maintenance window.

Fitting 18 TB of drives plus a second power supply takes more physical room than the 1U chassis on the 150 through 650 can hold. The extra rack unit is what makes the redundant hardware possible at this capacity.

Often, yes. With room for around 2,500 mailboxes, the 850 can consolidate what a few smaller appliances were handling separately, and federated search still lets you search across any older archivers you keep around during the transition instead of losing access to that history.

By appliance, same as every model in the lineup. One flat cost covers the 850 no matter how many of your 2,500 recommended users you're actually archiving, with the full feature set included and no per-seat charges.