Adding more storage to already modified iPod

I need some help. About 6 months ago, I had modified an iPod Classic (4th gen) from 60 GB to 128GB using two 64GB microSD cards and iFlash Dual.

I running out of space, so I tried to increase the storage to 320GB (a new 256GB + one of the existing 64GB cards). So, I replaced the 64-GB card on slot 1 with the 256GB card. When I turned the iPod ON, it showed me a warning - exclamation mark with folder icon. When I connected iPod to iTunes (on iMac), it was not recognized.

1. Why did I get the error and how do I resolve it?

2. Do I need to format the new card?

3. It is good assumption that the OS was on the card in the first slot?

Your insight will help a lot.

Thanks.