Luckily I figured this one ouf myself. Not 100% sure what it was that caused it (and other USB devices) to work more-or-less properly, but I am able to access the xD card from the Sandisk reader, as well as an SD card (simultaneously).
On the SD card, I was mounting it as> mount /dev/sdc /mnt/sd, which works fine.
When I went to try something similar on the xD card, it kept telling me it couldn't recognize the format of that device. Frustrated, I got all the data off the card and formatted it ON the Sandisk reader, and it seemed to work fine (using mkdosfs -c -C -n XDCARD /dev/sdb. note: I did this late last night a little intoxicated, so forgive me if I don't remember specifics
I remember it failed once, so perhaps the -C is not required, OR I had to format it as /dev/sdb1... regardless, it formatted the card). BUT, it failed to work in my Olympus digital camera since it wanted to reformat the card again, which I did. Took some pictures and put the card in the reader and tried to mount as before, again, failing.
So then I tried the partition thing again, and it WORKED!:
> mount -t vfat /dev/sdb1 /mnt/xd
Now, how did I know that it was /dev/sdb for the xD card? I tried to mount each SCSI device (except the DVD writers) using 'mount -t vfat /dev/sd[a-d] /mnt/<cardtype>, with the cards already inserted into the reader. I then examined the output in dmesg and noticed some differences in the errors being reported. At the time I only used two cards, the SD and the xD. I already knew which device my SD card was on, so it was a process of elimination to find the dmesg errors for the device, different from the other two. I won't go into specifics, but I believe it had to do with formatting/filesystem errors as opposed to 'make sure there is a disk in the drive'.
I decided to post this followup more for information reasons so that if anyone else has a similar problem with their Sandisk multicard reader, this might help point them in the right direction.