Embodiments of the present disclosure are directed to a system comprising a processing unit executing an enumeration process over a high performance I/O network having endpoint locations that can be populated with endpoint devices. The system further comprises endpoint emulators to represent endpoint devices at endpoint locations unpopulated during the enumeration process. The system facilitates accessing certain endpoint devices later inserted (e.g., hot plugged) into the endpoint locations that were unpopulated during the enumeration process. The endpoint emulators can be implemented using various techniques, such as cards plugged into card slots coupled to the endpoint locations, or emulation software and/or firmware exposed to the processing unit. In an embodiment, the system is a test system for testing endpoint devices (e.g., PCIe SSDs), which test system can facilitate hot plugging of units under test in test slots that were unpopulated during the enumeration process.