What are cold and hot-swap hard drives?
Hot swap hard drives are secondary hard drives that are used to backup the data of your original hard drive. In the case of hard drive corruption or failure, the secondary 'hot swap' hard drive can be swapped with the faulty hard drive without having to take the machine offline, heavily reducing the effect this has on the uptime of the equipment. It is necessary to have a RAID array if you are looking to employ hot swapping for your server solution.
Cold swap hard drives require the equipment to be turned off before the drive can be inserted. Whilst this will still reduce the downtime of your server, this option offers significantly less redundancy than a hot swap hard drive.