Hi @ModXE ,

May I know what OS you're using? Open Disk Management and check the followings:

• If Win7, assume hard drive in MBR table format, there should be a hidden drive named as SYSTEM_DRV
• If Win10, hard drive in GPT table format, there should be a hidden drive with no name and stated as "EFI system partition" in it's property.

Apart from system drive mentioned above (as well as the Windows OS drive of course), all the rest drive/partition can be re-allocated as any spare drive for user data storage. In conclusion, the "D drive" you found could be used as a normal spare drive.

That was supposed to be your data drive, not the recovery drive which is hidden. You must have a small C boot drive (ssd?) that will run out of file storage room. Use the D drive--or partition on large hard drives-- for all file storage--docs, music, photos, added programs, etc. Here's the manual--

https://www.dell.com/support/home/us/en/04/product-support/product/optiplex-7040-desktop/manuals 

Pick out your exact model. There are several 7040 versions. Open Disk Management to see a visual of all your drives.

To find Disk Management as mentioned above - Control Panel > Administrative Tools > Computer Management > Disk Management.

there is one drive hidden named recovery

Hi @ModXE ,

1. Login Windows, then hit the Windows button on keyboard.
2. Type-in "diskmgmt.msc" and hit Enter. It will then open the Windows Disk Manager.
3. Capture screen of disk manager and post here. I'll further let you know what to do with all those drive(s)/partition(s).

Also, let me know what Windows you're using now, Win7 or Win10.