Format your SD card on PC to FAT32 fileformat in normal way. Do not use any Amiga tools like HDToolBox, SDMount or Giggledisk.
Make sure you have:
DEVS:sagasd.devive
L:fat95
DEVS:DOSDrivers/SD0
DEVS:DOSDrivers/SD0.info
Make sure that your DEVS:DOSDrivers/SD0 mountlist uses partition geometry autosense, its contents must be like this:
FileSystem= l:fat95
Unit = 0
Flags = 0
LowCyl= 0
HighCyl= 0
Surfaces = 1
BlocksPerTrack = 1
BlockSize = 512
Buffers= 300
BufMemType= 1
BootPri= 0
Stacksize = 4096
Priority = 5
GlobVec= -1
DosType= 0x46415401
Device= sagasd.device