I have edited fstab and mtab to include my three external devices (all USB connections), then I can mount these devices in konsole.
I never touch /etc/mtab. The thing with USB devices is that they are dynamic. I have a SATA hard drive which is /dev/sda, when the OS is up and running, if I plug in my USB pen drive it will be /dev/sdb and because it only has one partition it is /dev/sdb1. Then after that I plug in my USB external drive with two partitions, they will be /dev/sdc1 and /dev/sdc2. But then the next day when the OS is running I plug in the external drive first, now it is /dev/sdb1 and /dev/sdb2, not /dev/sdc(x) like the day before. It's hard to define which is which when you have three USB devices powered on before booting up the OS.
I don't have Mepis, but I do have Debian, if I boot up the OS with my pen drive plugged in and the external drive powered on, neither will be auto-mounted as there is no entry in /etc/fstab, all I do is hit the "System" icon on the desktop and click on the appropriate device to mount it and view the contents in a file browser, it's that simple. And the device I mount first will be /dev/sdb and the next will be /dev/sdc.
Let's say for example, you have two external drives, one has ext3 file system, the other has Fat32, if one one of them is not powered on, how should the system react if the fstab entry for /dev/sdb is Fat 32 but the device is not present and it tries to mount /dev/sdc as /dev/sdb which has a different file system than what is defined in /etc/fstab, the OS will designate the device name based on the order in which it was mounted.
According to the quote I included at the top of this post, it appears even though there is an entry in /etc/fstab, you still end up mounting it manually via CLI, why don't you just hit the "System" icon and mount it with a mouse or mount it via CLI and forget about what's in /etc/fstab. The entries in /etc/fstab are there to have the OS auto-mount at boot-up, what's the sense having an entry in /etc/fstab if you're content with mounting them manually anyways?, which is what you say you're doing, and which is the way it is done with modern systems because of the vast array of USB devices available nowadays, which vary in file system structures greatly.