[Android Q&A, Help & Troubleshooting] rabinhood:
Android is just another linux distribution with fancy user interface, so all config files are kept in linux style. In every line all that comes after hash sign # is a comment. If you put hash as the first character, the whole line will be commented out, and won't be interpreted.

/system/etc/vold.fstab contains entries about mount points - in format that is described in the beginning of file: "dev_mount" command, space, label, space, mount point, space, mount options

Again, what you can do is to swich labels and mount points of sdcard and extsd. Your fstab would look like this:

*** FILE START ***

## Vold 2.0 fstab for HTC Passion

#

## - San Mehat (san@android.com)

##

#######################

## Regular device mount

##

## Format: dev_mount <label> <mount_point> <part> <sysfs_path1...>

## label - Label for the volume

## mount_point - Where the volume will be mounted

## part - Partition # (1 based), or 'auto' for first usable partition.

## <sysfs_path> - List of sysfs paths to source devices

######################

# Mounts the first usable partition of the specified device

#/devices/platform/awsmc.3/mmc_host for sdio

dev_mount extsd /mnt/extsd auto /devices/virtual/block/nandk /devices/platform/sunxi-mmc.2/mmc_host

dev_mount sdcard /mnt/sdcard auto /devices/platform/sunxi-mmc.1/mmc_host /devices/platform/sunxi-mmc.0/mmc_host

dev_mount usbhost1 /mnt/usbhost1 auto /devices/platform/sw-ehci.1 /devices/platform/sw_hcd_host0 /devices/platform/sw-ohci.1

*** END OF FILE ***

Of course to get access to system folders your device has to be rooted. But that's not all. Since vold.fstab is placed in /system/etc, your /system has to be mounted for read-write. You can achieve that from terminal emulator command line. First change user to root with "su" command. If your device is rooted, you should come with a request for root priviledges for terminal application. If you're root the command prompt will change from $ to #. You can always check the current logged in user with "whoami" command. It should provide username or user ID (UID). Root UID is 0.

To remount a partition with root rights type "mount -o remount,rw /system". If it returns no error, /system should be mounted in read-write mode for root user.

Unfortunately in my case the system refused to remount /system in rw mode, because, as D_rex noted, system partition in my device had filesystem errors, and a flag to mount read only when errors are found. To bypass this either try to modify flashable system image, as D_rex suggested, or, if you're lazy and just want to modify a live system, you can use my method with raw copying /dev/block/system partition to a file, modifying it, and then raw copying it back to /dev/block/system partition.

After this procedure /system should be accessible for write (you can also delete unwanted apk's from app folder to make some space, or add some ringtones to media folder etc).

But beware! Remember that you're messing with crucial system files, and if something goes horribly wrong, you might be forced to reflash stock system. Factory restore options won't work, since they don't touch /system, and all they do is they erase /data and /cache. So beforehand be sure you have original system sbf to flash, running flashing program on your PC, and know how to flash.