Which files/folders on my SDCard are unsafe to delete manually? I've had my Droid for nearly a year now, and in the process of installing and uninstalling apps, moving data to and from my SDCard, and all the various things one does with a phone the SDCard has become a little unruly.

It's time for a little file system maintenance.

It won't be difficult, but I only know enough about Linux file systems to be dangerous and I don't want to delete something important from my SDCard on accident.  I couldn't find anything on Android's site about how an SDCard is formatted, so is there anyone who can tell me which files/folders are off-limits or point me in the direction of a good resource?

Please note, I don't want to know which files/folders are safe to delete.  I know I can delete /Epicurious since I've uninstalled the app already, etc.

I'm running CyanogenMod 6.2.1 (based off Android 2.2.1) on my Motorola Droid.
I'm tempted to say everything is safe to delete.  It should be, with a storage device intended to be removed and used elsewhere.  But I know some apps will be unhappy.  So a good rule is not to delete anything that's clearly used by an app that you care about.

It's probably a good idea not to delete level 0 directories that start with a . such as /.android_secure/ (protected apps) either.  Those folders are generally used by the system and, while I don't believe Android ever stores anything important on the SD card (besides apps if you consider them important), there's probably no reason to delete them either.  /Android/ and /data/, /sd/ and /*.img, and /settings.db (if present) are also used by the system.  Those are app data, recovery images, and various settings respectively.

Also watch out for a /DCIM/ folder or similar, which may have any pictures you've taken.  Presumably you'd want to back them up.  Likewise with /download/ and /media/.

You'll want to watch out for /update.zip, /busybox, /clockworkmod/, and the like as well if you're rooted or have a custom recovery.

Note if, like me, you have "internal SD" as well as external, then most of the above is stored on the internal SD.  The external SD appears as /external_sd/ within the internal SD, or may be mounted at /emmc/ or similar.  You can apply the rules above to both storage areas.