Thanks. Good point, regarding the availability of the storage. Seems like NAS has a much higher chance of interruption (rebooting a router, etc, included).
Now that said, this is for a personal training lab, and the VMs themselves would run on the local disk. Most of them won't have 'external'/DAS storage, but I may need to add some extra files to Oracle's ASM with dummy environments. I don't mind backing those up, or losing some data, per-se... just would want to minimize the chances of that occurring. Seems like having an iMac or whitebox (ESXi) would be OK with DAS in this scenario, provided there is a UPS.
How do I add a vmdk to a vm that references some portion of DAS? Any reason to believe the interface (USB, Firewire, Thunderbolt) would matter?