[HTC Sensation] cipher75:
Yes, i experience that from time to time.

Only cure i found it to shutdown phone and start again, sometime i have to clear cache as well to get rid of those annoying FC.

I did get a logcat in order to understand what was going on. Here is what i read :

I/ActivityManager( 2237): Start proc android.process.media for restart android.process.media: pid=7602 uid=10005 gids={50005, 1028, 1015, 1023, 1024, 2001, 3003, 3007}
I/ActivityManager( 2237): Delaying start of: ServiceRecord{4257e5d0 u0 com.android.providers.downloads/.DownloadService}
E/AndroidRuntime( 7602): FATAL EXCEPTION: main
E/AndroidRuntime( 7602): Process: android.process.media, PID: 7602
E/AndroidRuntime( 7602): java.lang.RuntimeException: Unable to get provider com.android.providers.media.MediaProvider: java.lang.IllegalArgumentException: Can't obtain external volume ID for external volume.
E/AndroidRuntime( 7602):	at android.app.ActivityThread.installProvider(ActivityThread.java:4857)
E/AndroidRuntime( 7602):	at android.app.ActivityThread.installContentProviders(ActivityThread.java:4449)
E/AndroidRuntime( 7602):	at android.app.ActivityThread.handleBindApplication(ActivityThread.java:4389)
E/AndroidRuntime( 7602):	at de.robv.android.xposed.XposedBridge.invokeOriginalMethodNative(Native Method)
E/AndroidRuntime( 7602):	at de.robv.android.xposed.XposedBridge.handleHookedMethod(XposedBridge.java:547)
E/AndroidRuntime( 7602):	at android.app.ActivityThread.handleBindApplication(Native Method)
E/AndroidRuntime( 7602):	at android.app.ActivityThread.access$1500(ActivityThread.java:145)
E/AndroidRuntime( 7602):	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1266)
E/AndroidRuntime( 7602):	at android.os.Handler.dispatchMessage(Handler.java:102)
E/AndroidRuntime( 7602):	at android.os.Looper.loop(Looper.java:136)
E/AndroidRuntime( 7602):	at android.app.ActivityThread.main(ActivityThread.java:5081)
E/AndroidRuntime( 7602):	at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 7602):	at java.lang.reflect.Method.invoke(Method.java:515)
E/AndroidRuntime( 7602):	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:781)
E/AndroidRuntime( 7602):	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
E/AndroidRuntime( 7602):	at de.robv.android.xposed.XposedBridge.main(XposedBridge.java:126)
E/AndroidRuntime( 7602):	at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime( 7602): Caused by: java.lang.IllegalArgumentException: Can't obtain external volume ID for external volume.
E/AndroidRuntime( 7602):	at com.android.providers.media.MediaProvider.attachVolume(MediaProvider.java:5288)
E/AndroidRuntime( 7602):	at com.android.providers.media.MediaProvider.onCreate(MediaProvider.java:608)
E/AndroidRuntime( 7602):	at android.content.ContentProvider.attachInfo(ContentProvider.java:1591)
E/AndroidRuntime( 7602):	at android.content.ContentProvider.attachInfo(ContentProvider.java:1562)
E/AndroidRuntime( 7602):	at android.app.ActivityThread.installProvider(ActivityThread.java:4854)
E/AndroidRuntime( 7602):	... 16 more
W/ActivityManager( 2237): Process android.process.media has crashed too many times: killing!
I/ActivityManager( 2237): Killing 7602:android.process.media/u0a5 (adj 11): crash
W/ActivityManager( 2237): Unable to launch app com.android.providers.media/10005 for provider media: launching app became null
W/ActivityManager( 2237): Unable to launch app com.android.providers.media/10005 for provider media: launching app became null

For some reason, the system seems to loose ability to read UUID of the sdcard.

I found some posts on xda showing a similar issue, but those were connected to the fact that the sdcard was formatted in ext4fs. Mine is mounted as vfat, formatted in fat32.

Anyway i flashed the lastest ROM (15/02) yesterday, hopefully this issue won't appear on this ROM - previous was 08/02 where i experienced that several times.