I woke up this morning, unable to move my left arm at all.
I picked it up with my other arm, and dropped it back on the bed, felt as if it wasn’t attached. 30 or so seconds later I could move it, can anyone explain this?
I doubt this has anything to do with sleep paralysis.
It sounds more like your arm fell asleep due to you maybe lying on it or similar. It has happened for me a couple of times aswell.
Definitaley not sleep paralysis 
Like Tomas said, you probably were sleeping on it…
I’ve never heard of SP affecting just a limb, but even if it did, when you do get SP, you will know it for sure
I agree with Tomas and Dark Sider. It’s probably because you were sleeping on your arm. It has happened to me once.
What is sure, it has nothing to do with sleep paralysis.
Just can’t wait to meet the “Old Hag” (I’ll be scared as hell when it gets to that but anyway).
Old Hag is just an old LD4all legend, and you are more likely to meet the Loch Ness monster ! 
More info on this topic :
[url]What is the Old Hag nationality ?]
The body has a very intriguing safety mechanism when it comes to sleeping. If you get in a position that you cut off the circulation to a limb, in this case, your arm, the brain receives garbled signals, which you could imagine looking like black and white static on a televsion. If you can’t already, you will eventually feel these signals as pins and needles. When the messages are garbled, it wakes you up to prevent the loss of that limb. So you wake up and have to lift your arm (which can be kinda freaky cos it feels like someone else’s) to move it, but this is not sleep paralysis. Like they said, you’ll know it when it happens. 
Read up on my experienes here: [I am an "old hag" veteran)
When you wake up with a numb arm, it feels so weird. Scared the heck out of me when it (my numb arm) touched my other arm. felt like someone else had done it 
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I then had fun ‘shaking hands with myself’. It feels really odd when you’re only getting input from one of the limbs (try it yourself) .