I just inserted a Samsung 860 EVO SATA SSD into the M.2 slot and it seems to be working fine.
Performance is 73.7 MB/s for the SSD vs. 20.1 MB/s for the internal MMC.
SSD:
dd if=/dev/zero of=./zeros conv=fdatasync bs=384k count=1k
1024+0 records in
1024+0 records out
402653184 bytes (403 MB, 384 MiB) copied, 5.46706 s, 73.7 MB/s
MMC:
dd if=/dev/zero of=./zeros conv=fdatasync bs=384k count=1k
1024+0 records in
1024+0 records out
402653184 bytes (403 MB, 384 MiB) copied, 20.0723 s, 20.1 MB/s
