You rip your music to the PC, you have your .mp3 collection stored in My Music, when you select music to put on your iPod, it doesn't take it off your PC, it just takes copies of the music you select. The music on your iPod is still on your PC.
Backing up your music to an external drive is essential, in case of failure, virus etc. It's pointless putting music on your iPod and then deleting it from your PC. If you're worried about space, then ensure you have it all backed up before you delete.
It's very, very unlikely that your iPod will fail.