Over 4 years later my NAS finally broke. Can I fix it? 

Another great year of videos 😃 I have had a Raspberry Pi4 running now for 3 years none stop as a nas booting off a ssd plugged in to the usb and the nas drive also plugged into the other usb (no raid) Merry Christmas keep up the good work.

Suggest you find a SATA PCI card and cheap 128GB SATA drive ... that will resolve, even if no SATA native on mobo.