Hello,
Your C:/ drive is as you know your main drive, and allot of manufacturers are now partitioning a small portion of the hard drive to D:/ as a backup partition. They do this VIA things like RAID. Or its used as a tiny partition which is used for the recovery partition. If i were you i wouldnt touch it. Unless you partitioned it yourself and/or installed a second hard drvie or there is a second hard drvie inside the computer. :-). Also how big is the partition to find out right click it and press properties. If you dont have sufficient permissions its probably saying we dont want to let you in 'cos if this gets damaged you wont be able to restore your computer.
Hope this helps.
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Jamie kathys son.