Very surprisingly, this card with a Toshiba controller actually has 3 writeable erase blocks, not just one, and supports random access.
==> /sys/block/mmcblk0/device/block <== head: error reading `/sys/block/mmcblk0/device/block': Is a directory
==> /sys/block/mmcblk0/device/cid <== 02544d53413038471028cc2d7f00c200
==> /sys/block/mmcblk0/device/csd <== 400e00325b5900003a477f800a400000
==> /sys/block/mmcblk0/device/date <== 02/2012
==> /sys/block/mmcblk0/device/driver <== head: error reading `/sys/block/mmcblk0/device/driver': Is a directory
==> /sys/block/mmcblk0/device/erase_size <== 512
==> /sys/block/mmcblk0/device/fwrev <== 0x0
==> /sys/block/mmcblk0/device/hwrev <== 0x1
==> /sys/block/mmcblk0/device/manfid <== 0x000002
==> /sys/block/mmcblk0/device/name <== SA08G
==> /sys/block/mmcblk0/device/oemid <== 0x544d
==> /sys/block/mmcblk0/device/power <== head: error reading `/sys/block/mmcblk0/device/power': Is a directory
==> /sys/block/mmcblk0/device/preferred_erase_size <== 4194304
==> /sys/block/mmcblk0/device/scr <== 0235800001000000
==> /sys/block/mmcblk0/device/serial <== 0x28cc2d7f
==> /sys/block/mmcblk0/device/subsystem <== head: error reading `/sys/block/mmcblk0/device/subsystem': Is a directory
==> /sys/block/mmcblk0/device/type <== SD
==> /sys/block/mmcblk0/device/uevent <== DRIVER=mmcblk MMC_TYPE=SD MMC_NAME=SA08G MODALIAS=mmc:block
==> /proc/partitions <== 179 0 7639040 mmcblk0 179 1 7634944 mmcblk0p1
align 100663296 pre 545µs on 852µs post 734µs diff 213µs align 67108864 pre 545µs on 854µs post 734µs diff 215µs align 50331648 pre 543µs on 851µs post 732µs diff 213µs align 33554432 pre 543µs on 853µs post 735µs diff 214µs align 25165824 pre 545µs on 855µs post 733µs diff 217µs align 16777216 pre 547µs on 856µs post 740µs diff 212µs align 12582912 pre 543µs on 854µs post 736µs diff 214µs align 8388608 pre 542µs on 860µs post 751µs diff 214µs align 6291456 pre 541µs on 723µs post 731µs diff 86.7µs align 4194304 pre 550µs on 858µs post 741µs diff 213µs align 3145728 pre 545µs on 741µs post 732µs diff 103µs align 2097152 pre 529µs on 714µs post 718µs diff 89.9µs align 1572864 pre 529µs on 716µs post 720µs diff 91.5µs align 1048576 pre 545µs on 746µs post 733µs diff 107µs align 786432 pre 549µs on 748µs post 735µs diff 106µs align 524288 pre 548µs on 744µs post 732µs diff 104µs align 393216 pre 552µs on 748µs post 739µs diff 103µs align 262144 pre 548µs on 745µs post 741µs diff 101 align 196608 pre 549µs on 745µs post 738µs diff 101µs align 131072 pre 553µs on 749µs post 740µs diff 103µs align 98304 pre 546µs on 747µs post 735µs diff 107µs
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=1024 --erasesize=$[8*1024*1024] 8MiB 4.96M/s 4MiB 5.02M/s 2MiB 5.31M/s 1MiB 5M/s 512KiB 5M/s 256KiB 5M/s 128KiB 4.98M/s 64KiB 4.96M/s 32KiB 4.89M/s 16KiB 3.74M/s 8KiB 2.42M/s 4KiB 1.46M/s 2KiB 809K/s 1KiB 431K/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[8*1024*1024] --open-au-nr=2 8MiB 5.54M/s 4MiB 5.01M/s 2MiB 5.03M/s 1MiB 5.03M/s 512KiB 5.01M/s 256KiB 5.02M/s 128KiB 5.01M/s 64KiB 4.96M/s 32KiB 4.9M/s 16KiB 3.82M/s 8KiB 2.42M/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=2 4MiB 4.15M/s 2MiB 4.64M/s 1MiB 4.61M/s 512KiB 4.65M/s 256KiB 4.65M/s 128KiB 4.65M/s 64KiB 4.6M/s 32KiB 4.44M/s 16KiB 3.39M/s 8KiB 2.24M/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=2 --offset=$[17*1024*1024] 4MiB 3.6M/s 2MiB 2.27M/s 1MiB 2.72M/s 512KiB 3.03M/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=2 --offset=$[18*1024*1024] 4MiB 2.21M/s 2MiB 2.1M/s 1MiB 1.98M/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=2 --random 4MiB 5.03M/s 2MiB 2.69M/s 1MiB 3.15M/s 512KiB 1.43M/s 256KiB 1.75M/s 128KiB 1.89M/s 64KiB 1.37M/s 32KiB 1.2M/s 16KiB 1.19M/s 8KiB 613K/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=4 4MiB 3.23M/s 2MiB 4.27M/s 1MiB 1.45M/s 512KiB 628K/s ^Carnd@klappe2:~/git/flashbench$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=3 4MiB 3M/s 2MiB 4.58M/s 1MiB 4.57M/s 512KiB 4.55M/s 256KiB 4.57M/s 128KiB 4.54M/s 64KiB 4.52M/s 32KiB 4.37M/s 16KiB 3.34M/s 8KiB 2.2M/s
$ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=1 --random 4MiB 6.57M/s 2MiB 1.9M/s 1MiB 1.99M/s 512KiB 1.73M/s 256KiB 1.82M/s 128KiB 1.37M/s 64KiB 1.4M/s 32KiB 1.33M/s 16KiB 1.2M/s 8KiB 602K/s $ sudo ./flashbench --open-au /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=3 --random 4MiB 6.27M/s 2MiB 3.31M/s 1MiB 3.14M/s 512KiB 1.43M/s 256KiB 1.76M/s 128KiB 1.89M/s 64KiB 1.37M/s 32KiB 1.2M/s
$ sudo ./flashbench --findfat --fat-nr=6 /dev/mmcblk0 --blocksize=8192 --erasesize=$[4*1024*1024] --open-au-nr=1 --random 4MiB 752K/s 4.9M/s 2.86M/s 5M/s 5.01M/s 4.93M/s 2MiB 4.54M/s 4.53M/s 3.3M/s 2.19M/s 2.19M/s 2.32M/s 1MiB 4.03M/s 4.19M/s 2.33M/s 2.34M/s 2.34M/s 2.24M/s 512KiB 4.32M/s 4.17M/s 1.38M/s 1.38M/s 1.38M/s 1.38M/s