nprofile1qy2hwumn8ghj7un9d3shjtnddaehgu3wwp6kyqpqua7ld4as3dplpjfwdws9mk57y6cjrm4scxtcwhm825r7u02hul2qr6jqvx (nprofile…jqvx) Phil's Lab recently published a video showing how to build a USB Mass Storage Device from an STM32 and an SD card, which is the same rough framework as you'd want here, albeit slightly more complex because you'd want the STM32 to operate as both host and device. but proxying the general mass storage descriptor data and block transfers back and forth would be doable in almost the exact way he's showing here.
https://www.youtube.com/watch?v=aEwwQMdKd-c