All apps installed on my SD card have suddenly disappeared So this is very odd.
Last night my phone rebooted itself. Nothing unusual there - it does it from time to time. However, this morning when I try to read the news I get

&quot;Application is not installed on your phone&quot;.

So I check some more apps and lo and behold I get the same message. Any app that was installed on my SD card has gone - but only partially. The icon still exists on the &quot;desktop&quot; and the system name appears on the list of installed apps on the phone, but the Google Play store doesn't think they're installed - which is different to the symptoms described in these questions:

All apps moved onto the SD card have disappeared
My apps status suddenly become &quot;not installed on the phone&quot;, even the google play store!

which both state that the applications are still listed as installed in the Play Store.
Also - now I think about this a few apps had gone missing earlier. I have gaps in the icon field where I know there was an application, but because I didn't use it everyday I can't remember which ones :).
I've tried removing and reinserting the SD card, but that's had no effect.
When I reinstall applications that have local data and/or stored account details this information is still present. For example when I opened the Amazon MP3 player it didn't prompt me for my Amazon account details, but just showed the list of albums I've purchased. This implies that it's just the folder that hold the executables on the SD card that's been deleted/corrupted.
I've started reinstalling apps, but I'm concerned that this might happen again.
So does anyone have any ideas why this might be the case?
As Manishearth wrote, this might indicate trouble with the card (and very likely does). The boot you describe might have been "too abrupt", not cleanly unmounting the card and thus corrupting its file system; in which case the "good ol' chkdsk" should help (btw, Android ships with fsck for the very same task, which you could execute via a terminal app or adb shell – see Check and fix SD card errors within Android itself?).

On the other hand, it could also mean hardware trouble with the card, which even could be the reason for those reboots, reversing cause and effect: Not being able to read/write, or encountering too many "corrupt blocks", might have triggered some event (like re-mounting the card read/only). I'm not sure whether such would trigger a reboot (as stuff counted essential by the system usually doesn't reside on the sdcard), but it wouldn't be impossible. Thus trying a new card won't hurt.



PS – A little background on app2sd: As shipped with Android 2.2+, this only moves parts of the app to your card, while other parts remain on internal storage. Especially the app's data remain here (in /data), which is why after a re-installation of your apps they were already (or rather "still") available. For more details, you might wish to take a look at our app2sd tag-wiki.