Okay, so why is the computer saying to free up space?
Windows checks all mounted drives and warns if they are low on disk space.
if there is space on the drive?
But there isn't; there is space on C:, but D: drive is clearly low on space.
And how do I do this?
You don't. Windows suggestion to free up disk space on D: is stupid and pointless as freeing up any space means making it unusable.
Instead, you can change it so it does not have an assigned drive letter. This means it will be effectively invisible, including to the Disk space notification. Recovery Partitions are usually n ot much use once booted into Windows, and are usually used as part of the boot-up procedure.
You can remove the Drive Letter through Disk Management.
1. Start->Run (Or Windows Key+R) "Diskmgmt.msc"
2. In Disk Management, Find the Recovery Partition. Based on your earlier post it will be labelled "HP Pavilion" and will be assigned to drive D:
3. If you right-click this drive, there should be a "Change Drive Letter and Paths..." Option. Select it.
4. The dialog that appears should have a Remove button. Click it and confirm with the two prompts that appear, and the recovery partition will no longer be assigned a drive letter, so you will not receive warnings.