These are used to store your files, images and data. The bigger the disk drive, the more you can store on your server. Most small businesses will be fine with between 80Gb and 150Gb.
A budget server will come with a single disk drive of between, say, 80Gb and 300Gb. It will probably support an additional disk drive.
A mid-sized server will come with, say, 4 drives of between 80-700Gb each. You should expect the server to support at least six drives, preferably more. It should also support several options that allow the server to continue without interruption even if one of the disk drives fails.
You will need a CD/DVD drive in order to install software on the server. A writeable drive can be used for backup purposes.
If you get the option to add a diskette drive, you should take it - just in case. But you’re unlikely to need it.
For a mid-sized server, you should consider adding a tape backup unit and supporting software.
A budget server will fail if there is a failure in the power supply or a power interruption.
A mid-sized server will come with options for providing power in the event of a supply problem. You will need to consider how vulnerable you may be to power problems and configure your server appropriately.
This is likely to be a significant part of the overall cost of the server. Make sure it is included in the price you pay or that you budget for it as an extra. If you intend to run business software on the server, check what licensing you need with the developer or supplier. Ask about any other software you need such as email software like Microsoft Exchange or Lotus Notes.