How hardcore do you want your employee to be? If you want someone with some serious neck-beard credentials, I'd be asking about low level stuff like explaining how IRQ sharing works, or how the CPU scheduler works in NUMA architecture. Throw in some questions about TCP offloading or something similarly obscure like that.
If you're not after someone with such hardcore credentials, you can probably start with getting them to explain the differences between SAS and SATA (i.e. almost everything), or test their knowledge with some oldschool SCSI questions, like explaining what LVD is, or get them to identify the difference between PCI-E and PCI-X.
If they are going to be spending money or designing racks or networks, you might want to test their experience at this by asking if they have an estimate of the per-port costs for a 10GbE or 8Gb FC network. How much throughput do you get per-stream to an LACP or Etherchannel trunk?
For network design, ask them questions about OSPF and BGP. Ask them if there's ever a suitable scenario to use RIP. Which VLANs should you avoid using to keep compatibility with older equipment? What is 802.1QinQ?
Hardware is a large field - as large as programming perhaps, but with not nearly as many people in the field to write excellent interview questions like there are for programmers.