I need an external hard drive. How much GB do I need?

Thanks for the info and advice.  I have only a CD burner and I have backed up my music and photos on CD-R's.  The reason I'm thinking about an external hard drive is because I had a computer crash.  (It's a long story.)  I had not backed up a lot of WP documents - probably because I thought this could never happen to me!  So you say I should rethink the external hard drive.

While I have your attention and seeing that you do recording, I find that since the crash the MIDI function doesn't work.  First time that's happened.  On the Sibelius and theory softwares, it says no MIDI input is present.  I've plugged it in as I always have.  Am I missing a driver?

If you want to back up your "C" (internal hard drive) you will need one at least as large as the internal drive.  You will also need backup software that will backup or better yet make an exact "image" of the hard drive. 

Otherwise you can just backup your users data periodically and you don't need a special program for that, just the Windows Explorer. 

You don't want to just store your user files (e.g. music) on the external drive, you need it backed up someplace (murphy's law).  I do a lot of recording (I have a small studio) and I have CD's burned for all my music "masters", same way with my users data, I periodically back it up to a DVD (a DVD will hold all my users data except music files). You may want to consider DVD's (if you have a DVD burner) you can buy a lot of DVD's for the price of an external hard drive and I use DVD R's, not RW discs are they are unreliable. 

You can backup your user data files to CD-R's too, e.g. the "My Documents" folder.  It may take more than one CD-R but you can buy CD-R's for less than 25 cents each and if you watch the Sunday sales you can find them even cheaper, so it's a relatively inexpensive way to back up.  I used CD-R's before I got the DVD burner.  Same way with a DVD burner, I've seen them on sale for under $50 and you could replace the CD burner with a combination DVD/CD burner.

As far as the MIDI problem, it depends on what sound card (or on-boad sound module) that you have.  Some on-board sound modules do not have a MIDI Synth so you have to use a software synth such as the generic Microsoft Software Synth that comes with Windows or install a 3rd party software synth such as the Roland VSC3 series (which by the way comes with Band in a Box software).  If you got to the Sounds and Audio devices in the Control Panel you can see if there is a MIDI synth you can select - if it's not setup there any applications programs may not be able to detect a MIDI synth.  If you have a sound card such as the SoundBlaster series, they have built in hardware MIDI synth's and if you can't select one, with the Sounds and Audio Devices panel, you will probably have to reinstall the SB software to get the MIDI synths back.

Finally, if all you have is the Microsoft (software) synth, it's pretty bad and you may want to consider getting the Roland VSC3 (I think the 3 is the latest version - at least that is what is supplied with the latest versions of Band in a Box).  The Roland VSC MIDI synth is equivalent to the SoundBlaster hardware synths and does a decent job with the MIDI instruments.