Disk D is the partition to restore your computer to it's original state. Just as it was when you received it. You should not try to store anything else there. Back up your computer on dvd's or get an external disk for that purpose. Did you make the restore disks when you got your computer? If not you need to do so. Tell us make and model of computer and someone can help. If the hard drive fails you can't use the restore partition so you need the disks.

Before doing anything else you need to determine if you have already made recovery disks. If you have not done that then do so before any other actions are taken for safety sake. If you do not know how to proceed just Google 'create dell recovery disks' without the quotes for a tutorial.

Backing up onto the the D: partition is almost useless because that is simply another area on the same hard drive. In other words, if the hard drive fails you are still in big trouble.

Once those recovery disks are created you can play in the D: partition.

As stated the D partition is for your recovery data only, you should not put anything else in there or mess with that partition, should you need to recover the pc to original that is your only source if you have not made the recovery discs from that partition when you got the pc. Make those NOW. Leave D partition alone.
You want to back up the pc to some other source not the same hard drive. use dvds or flash drives or an external drive. The best way to do a back up is to do a full image backup a snapshot of the pc and the operating system. You can get a program to do that free if you do not have one.
This is one of them
Macrium Reflect.

Right click on a folder and select Properties, should show the date the folder was initially made.

OK. I didn't know "D" was a logical drive, since the display from clicking on the Computer icon seemed to be showing it as a separate physical drive. I assumed too much from that display. From my list of folders that are on D, it looks like the folders for recovery are there. I did a System Restore after the incomplete backup was attempted, and the restore completed normally. Would that indicate that D is OK?

Thanks for the helpful explanations. I have a set of discs that came with my computer, one for the operating system, Dell utilities, a sealed Microsoft Windows disc, others. From what I see on the Dell site, it was with Windows 7 they stopped sending discs. There were no instructions to make the restore discs when I installed my computer. BUT, I'm going to make a new copy anyway. Is the snapshot of the system preferable? Is there a way to estimate how many DVDs I'll need?

Again, thanks all.

You may find the answer on this page of the Windows 7 forums. If you haven't partitioned a drive you probably wouldn't know that each partition is given a different drive letter and treated as another drive.

I did a System Restore after the incomplete backup was attempted, and the restore completed normally. Would that indicate that D is OK?

No a system restore only takes you back to the date the restore point was created. The recovery disks you have will take the computer back to the way it was the day you got it home.

To do a full backup using Windows 7 backup feature click start
click Control panel
click System and Security and in the new window you will see where to click to proceed. As I do not back up very often I cannot remember if it tells you how many DVDs you need, likely not less than 3 or more than 5.

System Restore is usually from your "C:\System Volume Information" a hidden folder by default.

Sounds like your D has the OEM install files on it.. to revert your computer hard drive back to Out of the Box new condition. If you didn't delete any of it :)

.... An image of your C drive made by a program like Macrium will usually compress to about half the total used bytes that that drive shows as currently used in windows.
Usually too big to image to DVD's unless you don't mind spending the time it would take to do,,, and of course the cost of several DVD's.
My C drive is small compared to most peoples on win7 and shows 60gb used .. divided by about 4.7gb per DVD.. thats a lot of DVD's and lots of time sitting there changing those disks.

fwiw
Paid about $11 delivered from newegg for a usb hard drive enclosure and stuck an old laptop hard drive in it & use that here to save system images to weekly. Think DVD-R's cost me about $0.40 each in a pack of 50 including tax from Office Depot.

I'd recommend that you purchase an external hard drive to store your backups. First, do a complete image backup, and then you can do incremental backups if you wish. Lots of information if you use Google to search. Raven mentioned Macrium as one free backup program. Another good one that I use is EaseUS ToDo backup. I don't bother with incremental backups but make a full image backup about every two months right after the MS updates come out. This way I always have a complete version of the latest documents, programs, OS and updates that I can use to restore in case of a catastrophe. I have done a restore that way, and it saved me much work. There are also paid programs, but why pay when the freebies work so well?