I have two memory sticks with my music on them for my Bolero and today,for some reason,they aren't working and being recognised (yesterday no issues).

 

My smartphone links to Android Auto easily,charges the phone and works fine,so the USB port isn't at fault.

 

The memory sticks are ok on my desktop.

 

What could be the problem?

I might guess at the wrong format of the USB. It probably needs to be FAT as opposed to NTFS.

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But they've been working fine for the last 3 weeks?

 

I notice that there's an option to 'safely remove' on the Bolero,which I've not been using. Is that critical? (although I've been happily NOT safely removing them and they've still worked.

It might be, to ensure data integrity you should always use a safely remove option if it's available. Shouldn't really matter on a read only connection that's in your car but you never know. It should also be possible to to protect your SD card by moving the tab on the card to  'Lock' making it 'Read only'