I thought I would share my findings to help others with the same issue.
Two days ago I kept catching glimpses of a card image in the notifications at the top of the screen, but it would only be for a second so I could not see what it was trying to tell me. But had a quick look and sd card seemed fine and all data was present on handset. A couple of hours later it popped up with "SD Card Unexpectedly Removed". I tried ejecting and reinserting,restarting device and turning off and removing battery all to no avail. So I did a quick search which showed this seemed to be quite a common issue, after reading many threads with people saying the sd card is corrupt or network providers saying it was software and to do a restore. I decided before all that hassle I would open the phone and check the circuitboard expecting it to be a dry solder joint. One thing I will point out that was different to some other users is the fact my sd card was not visible with the phone connected to the computer, so this may not solve all related issues.
Turn off phone, remove the battery, SIM card and sd card. Remove all screws except the one by the 3.5mm jack. Gently pull the white plastic trim that surrounds the card holders and battery but the top part with speaker and camera does not need removing. Once removed if you look to the left of the sd card holder you will see the ribbon that connects the sd card to the main circuit board and at the end of the ribbon is the connector. Mine was disconnected and was loose, even if yours is not loose it would be worth gently pulling it off and reconnecting it. Then reassemble in the reverse order.
This was a nice surprise in the sense it was a very simple fix. However when I mounted the sd card even directly in my pc it was not found, I assume that the loose connector floating about had probably shorted its connections and caused an issue with the sd card. After a handfuls of attempts at mounting it in my pc it was finally detected and I backed up all the data and reformatted the sd card, then copied the data back,mounted it in my phone and was happy to find it all working as expected.
I hope this information will help some of you out, as it was a simple fix to a simple issue and results from searching had no solutions, just lots of assumptions.