Ok I am pleased to report that i found out what happened and have fixed it and the SD card now works perfectly again in the TT.
Before I updated the maps on this card i had reformatted and checked everything in disk management and all was well. putting back the navcore and updated maps it all was fine and worked. Only then did i copy the BL to the SD card, flashed and then the SD would not load in the TT but others would. Also it worked on other TTs i tried (but they all also had internal SD cards as well as slots). I had reformatted and tested the SD partition finding no problems, but it still hadn't worked.

I had forgotten to re check in disk management. When I had flashed the TT had created a 13MB unallocated partition on the SD that was not there before flashing. You can not do this in windows 7 not without special tools, and it was not there before I put it n the TT to flash. I had to use another tool to low level format the SD card to remove the partitions (as its not possible in windows on SD cards). And now its re formatted and files returned it works again.
So the TT had damaged the card during the flash, but now all repaired.

Here's a pic of the card before repair
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