I've just been playing with my old X'sDrivePro on my newly acquired (it's not really "new") 3100, and it readily mounted the HD without incident but it wouldn't let me mount an SD card inserted in it.
So I did a little digging.
Multi-card/HD devices show up as scsi devices in linux, which is good, and they give each separate slot a different LUN. Linux only attaches to the first LUN automatically. Which is bad. My device has the HD as LUN 0, so I have no problem getting it without further intervention, but I guess the vosonic has one of the card slots as LUN 0 instead.
So, the magic incantation to attach another LUN is:
echo 'scsi add-single-device 0 0 0 1' > /proc/scsi/scsi
where the "0 0 0 1" is host, channel, id, lun. Do a 'cat /proc/scsi/scsi' to get the first three numbers (probably 0 0 0 anyway), you should already have an entry with LUN 0. You would then keep incrementing the last number until they stopped working.
Or, you could do it the simple way and use the script I've attached. It comes from
http://www.garloff.de/kurt/linux/scsidev/, and I've modified it slightly to avoid problems with busybox. Unzip it, chmod +x it, then:
./rescan_scsi_bus.sh -l
It will then scan and attach to whatever it can (the -l is important! it wont scan LUNs >0 otherwise!)
Doing a "cat /proc/scsi/scsi" now should show you several new devices, which you can mount using /dev/sdb1, /dev/sdc1 etc.
My X'sDrivePro had devices on 4 LUNs, meaning /dev/sda1, /dev/sdb1, /dev/sdc1, /dev/sdd1. pdaXrom doesn't have the /dev/sddX devices, and guess which one my SD card was mapped to! If you need them, creating the devices is easy.
mknod /dev/sdd b 8 48
mknod /dev/sdd1 b 8 49
mknod /dev/sdd2 b 8 50
mknod /dev/sdd3 b 8 51
mknod /dev/sdd4 b 8 52
mknod /dev/sdd5 b 8 53
mknod /dev/sdd6 b 8 54
mknod /dev/sdd7 b 8 55
mknod /dev/sdd8 b 8 56
mknod /dev/sdd9 b 8 57
The numbers are important. The first is always 8, the second starts at a multiple of 16 (sda is 0, sdb is 16, sdc is 32, sdd is 48 etc) and the numbered devices go up incrementally from there. I only actually needed sdd and sdd1, but it's better to be complete