The simplest being choosing fixed or hot-plug drive mounting. There's a neat slide-out tray system for 3.5" drives that allows hot-plug access for densely vertically packed drives that could be adapted to 2.5" drives.
There seem to be roughly three hardware approaches, all of which have different performance goals, different MTDL intents and :
- Single board, 'back blaze' style. Capacity-optimised, low external bandwidth.
- High Availability, dual motherboard, dual-ported local Embedded RAID Controllers, zero contention internal fabric, high performance external interfaces, additional Flash memory, NVRAM memory
- Mid-scale server. Single board, fast external interfaces, Embedded RAID Controllers, low contention internal fabric.