Drobo Mini / Drobo 5D cannot be an iSCSI target; they don't have any Ethernet interfaces - only Thunderbolt and USB 3.
There's a Drobo B800i, but it costs $2500 barebones. It has two Gigabit Ethernet interfaces, and the only protocol it supports is iSCSI. So, it's designed specifically to be an iSCSI datastore with RAID (up to 8 HDs or SSDs). This is way too expensive, and also, it's quite noisy.
I'm interested specifically in Drobo Mini /Drobo 5D, which are purely Thunderbolt and USB 3 devices.