I've been struggling with the SAME70-XPLD board to use the SD card through the HSMCI peripheral. The throughput i reach is about 420 kBps.
I have tried the ASF example code with about the same throughput (it was slightly slower in my memories) though most of my code is based on the example.
Im using FATFS on top of the HSMCI interface software but that doesnt seem to influence the transfer speed that much.
When looking at the signals on a oscilloscope it seems that DAT0 remains low for a while after each written packet thus indicating the card is busy (?) but that seems unlikely to me since this is a quite low write speed for a new 2018 SanDisk SD card. A screenshot of the scope is attached here brown is the clock (50MHz), red is the command line and yellow is DAT0. When DAT0 is high a new packet is being composed so that delay makes sense to me.
The Code is added to the attachments
Hope someone of you can help me, thanks in advance!