A computer tower needing an external power supply. There is nothing behind the power supply "cutout". Basically a laptop motherboard in a case. Also, the card slots aren't removable. : r/techsupportgore
External 5v/12v Ac Powerful Adapter Usb To Ide+sata Power Supply Adapters Converter Cable For H-dd/hard Drive/cd-rom - Pc Hardware Cables & Adapters - AliExpress
Mackie External Power Supply for Select Ultracompact 0028090-00
External Power Supply | DSAN
12V Seagate 9SEAN2-500 External hard drive replacement power supply | eBay
Phobya external PSU 230V to 4Pin Molex 70W including Euro/UK plug | 230V accessories | Accessories | Water cooling | Aquatuning USA
AC Adapter Power Supply for Western Digital WD S018EM1200150 External Hard Drive | eBay
65 Watt External Power Supply (all-voltage) – ZVOX Audio