performance - Other than portability, are there any other advantages (or disadvantages) to storing apps on external storage?

Are there any particular advantages/disadvantages to storing apps on the external storage of an Android device? As I have a Nexus S, I can't think of advantages for me as I can't remove the storage, also the internal storage is more than enough for my needs.

I noticed that on phone start up, when I had all my apps moved to USB storage it was quite sluggish to register all the apps on the menu/home screens.

I also read somewhere that having a wallpaper set from a photo that was stored on the external memory actually drained quite a bit of battery too with all the reads (although I think this may have been a bug).

Answer

Advantages:

• Extend the available space on internal memory for installing more apps.
  Although some parts of the app still stay on the internal memory (data and settings,) it still frees up a significant amount of space. For older handsets with 150MB of free space (G1, MT3G, etc.) this is a huge improvement. For owners of these devices this advantage alone outweighs any disadvantages that this method brings.

Disadvantages:

• Cannot use apps moved to SD Card if it's unmounted.
  Kind of self-explanatory: if you un-mount the SD Card for removal, the apps that reside on it need to be stopped and um-mounted as well to prevent crashes and data loss if you do remove the card.


• Inability to use widgets or sync adapters when you move those apps to SD Card.
  This is a design limitation that stands from the above point. Since the apps stored on SD Cards need to be able to be removed on demand, things like widgets or apps that hook into your account settings (e.g. Skype, LinkedIn) will crash, with no easy way to re-start them. So Google made a compromise by requiring these apps to stay on internal memory. Even if you do manage to force them to the SD Card (some custom ROMs allow this), you simply won't be able to use them.


• Longer boot-up and shut-down times when the system mounts/un-mounts all of the apps stored on SD Card.
  Apps that were moved to SD Card are not stored in plain sight to prevent rampant piracy. Instead, Android sets aside some space and creates a virtual partition for each app that you move. This means that at the initial boot-up, all these virtual partitions need to be mounted before apps stored on them can be accessed. The same holds true when you turn the handset off - each virtual partition is un-mounted before other shutdown processes can happen. Depending on the number of installed apps this can take an extra minute or even longer for the system to "stabilize" upon boot-up (and about 20 extra seconds for shutting down.) Once this happens though, launching these apps doesn't take any longer than those on internal memory.


• Extra wear on the SD Card.
  This point is debatable, since writing to SD Card (which causes wear) doesn't happen that often. And besides, the SD Cards these days are cheap enough to not worry about their life time.


• Apps require an update to support this functionality.
  
  Ability to be moved to SD Card is not automatically granted to older apps. Developers must change their apps to support this feature and issue an update via Market (or whatever distribution method they use.) Some older apps may not be supported any longer, and so may never get this feature. Although some custom ROMs can override this, for most people that are running stock firmware, this is a problem.

For newer devices with gigabytes of internal memory moving apps to external storage is not necessary, since space is not an issue. However for handsets with low memory it's an invaluable tool that extends their shelf life for their owners.