I think that you should contact the HxC support :
http://torlus.com/floppy/forum
http://hxc2001.free.fr/imiel.htm
Regarding your issues, you may have a power supply problem with the Emax, or you have got some very bad Sandisk-clone sdcard...
About SDCard: Like all flash medias (USB key,CF,...) there is a lot of counterfeit cards everywhere :
FAKE SanDisk Ultra Secure Digital Cards Exposed
So get the good ones, and they will works many years without issue...
About write problems : It seems that there is something special with the Emax : You must use the right OS to avoid these write problems.
For the Read/write speed : The floppy bus speed cannot be increased... So no improvement possible here. The limitation is the Emax floppy controller.
Contact me at this email http://hxc2001.free.fr/imiel.htm if you want to solve these issues.
thanks
jf