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Answer to: Up arrow key not working after cleaningI know that is not really a answer to your question but i recommend to use a barely wet cloth to gently remove dirt from the keyboard, only water no cleaners or other fancy stuff.
Answer to: head phone jack stuckYes you can replace the headphone jack but its one piece together with the hold switch. And this iPod model is quite hard to open. See here: iPod Classic Headphone Jack & Hold Switch Replacement
Answer to: How to install dual hard drives, solid state drive and Serial ATA?maybe this helps: How to add one or two SSDs without interfering with fan issues