Hey
I am not sure how you use your memsticks but I always do:

mount -t vfat /dev/sda1 /mount/memstick

It worked without any problems on slack and fc2.

Give it a try

FimDan

Thing called udev creates the device sda1 automagically when kernel detects an usb device. Actually it goes like this: when you plug your usb stick in, the kernel calls little script called /sbin/hotplug. This script in turn calls progs linked in /etc/hotplug.d. There you'll find link to udev. When udev is called it scans the sysfs pseudo filesystem (similar to proc) for new devices and creates the devices according to rules found in /etc/udev/.

Now at some point there is a glitch (as you might have noticed :) )..

When your usb stick is not being recognised do this: run dmesg | tail -n 10 in a console. It prints last 10 lines your kernel has printed. Now plug your usb stick in and run the previous command again. Compare these two outputs. Do you see anything about usb mass storage? If you do then kernel is propably working correctly (unless you see an error message instead of just normal notice).

If the kernel is a ok, then it is probably some problem in the hotplug scripts or udev rules. Try to find for a similar bug in Mepis mailing lists or install updates for the hotplug/udev scripts.

You could write a hotplug script to do it automaticly as soon as a usb mass storage device is detected.