Ask on gamr13's discord channel, iirc, there's a help chat in there.I test the cdi from the internal disk, damnit ,it worked.
And I downloaded the log while error with external disk pasted below. I think maybe it is related to the core vfs support function.
[libretro INFO] core/hw/mem/_vmem.cpp:494 N[VMEM]: Info: nvmem is enabled, with addr space of size 512MB
[libretro INFO] core/hw/mem/_vmem.cpp:589 N[VMEM]: BASE 00007ff4284f0000 RAM(16 MB) 00007ff4344f0000 VRAM64(8 MB) 00007ff42c4f0000 ARAM(2 MB) 00007ff4484f0000
[INFO] [Content]: Core does not support VFS - copying to cache directory.
ERROR Content: 无法读取游戏文件 "D:\roms\Dreamcast\斑鸠.cdi". (during copy read or write)[/ERROR]