In the 1990s, Dell switched from using primarily ATX motherboards and PSU to using boards and power supplies with mechanically identical but differently wired connectors. This meant customers wishing to upgrade their hardware would have to replace parts with scarce Dell-compatible parts instead of commonly available parts. While motherboard power connections reverted to the industry standard in 2003, Dell continues to remain secretive about their motherboard pin-outs for peripherals (such as MMC readers and power on/off switches and LEDs).
With what kind of parts weren't consumers able to upgrade their Dell systems with? (If the question is unanswerable, say "unanswerable")
unanswerable