What files do I need for a backup? Only wallet.bin?
I'm backuping the whole set of wallets' files - just in case.
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I tried restoring from the key file and it sort of worked, but it lost (and did not restore once syncing to the block chain) the record of which outputs had been spent. Trying to send coins after that caused a failure due to double spend.
Not good. Back up your entire wallet, not just the keys.
Before restoring from the key file you should delete wallet.bin from the folder. Did you?