Hiding the D: drive (the restore drive) from Windows

Most computers today come with the hard drive split to look like 2 hard drives.  The reason manufacturers are doing this is because one is used for the computer and the other contains all the files necessary to restore the PC to factory settings (reimage the pc).  Since that partition is usually sized so that it doesn't have that much free space, nobody ever uses it as extra storage space.  As a result, it's pretty much a useless drive until you need it.  So, why not hide it from the operating system?  It could be done, but the manufacturers don't want to take the time to do it.
Taken from howtogeek.com:

Open regedit and navigate to HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
The NoDrives key most likely does not exist by default, so you’ll need to create it with right-click \ new 32-bit DWORD and name it NoDrives.
This value is a 32 bit number, and the bits are arranged in reverse order with a value of 1 hiding that drive. For example, if we wanted to hide drives A: and F: we would arrange it like this:

Z	Y	X	W	V	U	T	S	R	Q	P	O	N	M	L	K	J	I	H	G	F	E	D	C	B	A
0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	0	1	0	0	0	0	1

Converting 100001 to decimal we end up with a decimal value of 33 or a hex value of 0×21, so if you double-click on the key in the registry editor, choose Decimal and then enter 33 into the value field.
In order to see these changes, you’ll need to restart explorer.exe, which you can do easily from Task Manager or the longer way by just logging off and back on.