[General Questions and Answers] ciubexx:
A possible solution.

I have this issue too, but in my case the issue was caused by a wrong formatted card:

I created by mistake a logical partition, instead primary partition on my card.

Is important also to check in this case your SD card, try to power off, remove it the SD card from the device (phone/table) and power on the device without the SD card.

If that is fixe it, then that is the cause in your case too.

In my case is about an Sony Xperia V (LT25i) and below is a a piece of my log cat:

05-21 18:25:13.552: W/MountService(829): getVolumeState(/storage/sdcard1): Unknown volume
05-21 18:25:13.552: E/AndroidRuntime(1393): FATAL EXCEPTION: main
05-21 18:25:13.552: E/AndroidRuntime(1393): java.lang.RuntimeException: Unable to create service com.sonyericsson.usbux.service.UsbService: java.lang.IllegalArgumentException
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.app.ActivityThread.handleCreateService(ActivityThread.java:2651)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.app.ActivityThread.access$1600(ActivityThread.java:150)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1380)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.os.Handler.dispatchMessage(Handler.java:99)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.os.Looper.loop(Looper.java:213)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.app.ActivityThread.main(ActivityThread.java:5225)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at java.lang.reflect.Method.invokeNative(Native Method)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at java.lang.reflect.Method.invoke(Method.java:525)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:741)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at dalvik.system.NativeStart.main(Native Method)
05-21 18:25:13.552: E/AndroidRuntime(1393): Caused by: java.lang.IllegalArgumentException
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.os.Parcel.readException(Parcel.java:1448)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.os.Parcel.readException(Parcel.java:1395)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.os.storage.IMountService$Stub$Proxy.getVolumeState(IMountService.java:241)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.os.storage.StorageManager.getVolumeState(StorageManager.java:564)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at com.sonyericsson.usbux.service.SdCard.getState(SdCard.java:94)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at com.sonyericsson.usbux.service.UsbNotifications.<init>(UsbNotifications.java:60)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at com.sonyericsson.usbux.service.UsbService.onCreate(UsbService.java:214)
05-21 18:25:13.552: E/AndroidRuntime(1393):	at android.app.ActivityThread.handleCreateService(ActivityThread.java:2641)
05-21 18:25:13.552: E/AndroidRuntime(1393):	... 10 more
05-21 18:25:13.582: I/ActivityManager(829): Start proc com.sonymobile.gpssatellitesnotification for broadcast com.sonymobile.gpssatellitesnotification/.GpsSatellitesListener: pid=1523 uid=10058 gids={50058, 1028}
05-21 18:25:13.612: I/ActivityManager(829): START u0 {act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10000000 cmp=com.sonyericsson.initialbootsetup/.HomeActivity} from pid 0
05-21 18:25:13.622: W/ContextImpl(829): Calling a method in the system process without a qualified user: android.app.ContextImpl.bindService:1481 android.bluetooth.BluetoothA2dp.<init>:164 android.bluetooth.BluetoothAdapter.getProfileProxy:1169 android.media.AudioService$AudioServiceBroadcastReceiver.onReceive:4280 android.app.LoadedApk$ReceiverDispatcher$Args.run:770
05-21 18:25:13.622: V/WiredAccessoryManager(829): init()
05-21 18:25:13.812: I/ActivityManager(829): Start proc com.sonyericsson.android.bootinfo:remote for service com.sonyericsson.android.bootinfo/.MiscTaService: pid=1553 uid=1000 gids={41000, 1015, 1028, 3002, 3001, 3003, 3007, 1021, 3004, 3005, 1000, 3009, 1023}
05-21 18:25:13.812: I/ActivityManager(829): START u0 {act=android.intent.action.MAIN cat=[android.intent.category.HOME] flg=0x10008000 cmp=com.sonyericsson.setupwizard/.SetupWizardEntry (has extras)} from pid 1365
05-21 18:25:13.853: I/ActivityManager(829): Start proc android.process.acore for content provider com.android.providers.contacts/.SemcCallLogProvider: pid=1563 uid=10010 gids={50010, 1028}
05-21 18:25:13.863: I/ActivityManager(829): Start proc com.google.android.gms for broadcast com.google.android.gms/com.google.android.location.internal.NlpNetworkProviderSettingsUpdateReceiver: pid=1569 uid=10050 gids={50050, 3003, 2001, 1007, 3006, 1028, 1015, 3002, 3001}
05-21 18:25:13.913: I/ActivityManager(829): START u0 {cmp=com.sonyericsson.setupwizard/.screens.ImportantInformationScreen} from pid 1286
05-21 18:25:13.943: E/ActivityThread(1208): Failed to find provider info for com.sonymobile.settings.illumination
05-21 18:25:13.973: E/ActivityThread(1208): Failed to find provider info for com.sonymobile.settings.illumination
05-21 18:25:13.973: E/ActivityThread(1208): Failed to find provider info for com.sonymobile.settings.illumination
05-21 18:25:14.263: W/MountService(829): getVolumeState(/storage/usbdisk): Unknown volume
05-21 18:25:14.263: E/AndroidRuntime(1117): FATAL EXCEPTION: main
05-21 18:25:14.263: E/AndroidRuntime(1117): java.lang.RuntimeException: Unable to create service com.android.systemui.SystemUIService: java.lang.IllegalArgumentException
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.app.ActivityThread.handleCreateService(ActivityThread.java:2651)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.app.ActivityThread.access$1600(ActivityThread.java:150)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1380)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.os.Handler.dispatchMessage(Handler.java:99)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.os.Looper.loop(Looper.java:213)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.app.ActivityThread.main(ActivityThread.java:5225)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at java.lang.reflect.Method.invokeNative(Native Method)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at java.lang.reflect.Method.invoke(Method.java:525)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:741)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at dalvik.system.NativeStart.main(Native Method)
05-21 18:25:14.263: E/AndroidRuntime(1117): Caused by: java.lang.IllegalArgumentException
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.os.Parcel.readException(Parcel.java:1448)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.os.Parcel.readException(Parcel.java:1395)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.os.storage.IMountService$Stub$Proxy.getVolumeState(IMountService.java:241)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.os.storage.StorageManager.getVolumeState(StorageManager.java:564)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.systemui.usb.StorageNotification.checkUsbStorageState(StorageNotification.java:107)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.systemui.usb.StorageNotification.<init>(StorageNotification.java:96)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.systemui.statusbar.phone.PhoneStatusBarPolicy.<init>(PhoneStatusBarPolicy.java:182)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.systemui.statusbar.phone.PhoneStatusBar.start(PhoneStatusBar.java:497)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at com.android.systemui.SystemUIService.onCreate(SystemUIService.java:100)
05-21 18:25:14.263: E/AndroidRuntime(1117):	at android.app.ActivityThread.handleCreateService(ActivityThread.java:2641)
05-21 18:25:14.263: E/AndroidRuntime(1117):	... 10 more

@nandude I suspect the SD card in your case too, because when you connect the phone to PC normally the SD card is mounted on the PC and is not scanned by the phone, but once you disconnect your phone from PC the SD card is scanned again. Try to change how is attached the SD card to PC (MTP or MSC).