How to format /storage/sdcard0 in ext4 I tried to create a symbolic link on the "internal SD card" of my Galaxy SD2. It failed with "permission denied" and I quickly realized this file system is vfat.

How can I reformat /storage/sdcard0 with a modern file system instead, like ext4?

Are there any drawbacks if I do this?
Possible drawbacks
As for the &quot;drawbacks&quot; part, there are different things to consider:

Windows cannot read ext4
Mac/Linux/*nix should have no trouble with it
Using MTP, Windows won't have any trouble either
If your ROM supports ext4, there should be no local problem
You might want to switch to a different ROM one day -- that ROM might not support ext4 (unlikely, but not impossible)

There are several situations to consider, as well:

If you one day get trouble with your card which cannot be solved &quot;locally&quot; (e.g. the card no longer mounts locally, or cannot be mounted via MTP/UMS, or you need to do a recovery of accidentally deleted files), you might need to attach it to your PC using a card reader (ext4 ↔ Windows: Fail!)
If you lend your device to somebody only having Windows, he might encounter problems
If you one day sell your device, you have to keep this in mind, too

Though for the last two situations, the solution might be simply to remove/replace the card, or re-format it (in the last case).
How to do it
Still here? OK, so the drawbacks seem to be negligible. There are multiple ways of how to get your card formatted with any file system:

use a card reader on your Linux-PC or Mac, and the mkfs command with the -t ext4 option (and additional parameters)
use a terminal app to do this locally on your device (see my answer on How can I format a flash drive using terminal emulator? for details)