60 disks in 4U!
I admit they are special racks (they are nearer 30" wide and goodness knows how deep), but in the IBM P7 775 supercomputer disk enclosures, you can get 384 2.5" disks in 4Us of vertical space.
On more mainstream systems, and having used dual-connected SAS drives for about the last 5 years, I will say that the biggest problem here is the repair of a failed expander card in the disk drawer. The problem is that although they are redundant, so the loss of a SAS expander does not stop the service, the repair action is not normally concurrent. This means that you have to take an outage in order to restore the full resilience, even if you have the connected to dual servers unless you have the data moved or mirrored to disk in another unit. The saving grace is that you can plan the outage, but you have to be careful if you are wanting very high availability.
I learned this the hard way when planning for service work in what had been delivered as a totally redundant system. A bit embarrassing when you end up having to stop all of the workload on a top 500 HPC system just to carry out the work for a single expander card (no, I was not responsible for the design, I only help run it, and it could have been mitigated with a bit more thought)
By the way, this dual connectivity is not a new thing. IBM's SSA disk subsystem also had dual connectivity for both disks and servers back in the mid 1990's. Very popular for HA/CMP configurations, and allowed for 48 disks in 4U of space.