Sep 12, Update: Windows 10 Free Upgrade is still available-ZDNET
Windows 10 free upgrade offer ends in this month, July.2016. When you upgrade, "Windows need more space" the error message may show if you have low disk space on system partition, C Drive, to be more specific. How to fix not enough disk space for Windows 10 upgrade?
Step 1: Run Disk Management by input diskmgmt.msc in WIN+R run box
Step 2: Click on D partition to Delete Volume in the right-click menu
NOTE: Do a backup to make sure data in a safe place. Delete Volume will erase all data on that partition.
Step 3: Click YES to proceed, then D partition will become unallocated space
Step 4: Right-click on C Drive, select Extend Volume in the menu, and then follow Extend Volume Wizard to extend C Drive.
NOTE: File System should be NTFS. FAT32 is not supported in Disk Management.
That's the steps in Disk Management to Extend C Drive with data loss in D Drive.
Disk Partition Expert can resize/move volume without data loss in D Drive and make unallocated space extended to C Drive in quick time. Do as follow.
1. Download, install, Run Disk Partition Expert, Click on D Drive > Choose Resize/Move Volume.
2. Shrink D Drive by dragging the handle on left side, which space will become unallocated in the disk map below.
3. OK > click on C Drive, Resize/Move Volume, Drag the handle on the right side to extend C Drive.
4. OK, Commit, Done
If you have free space on E partition or any other partition, you can move free space from the large size partition near to small size partition and do the extend job.
In the whole operation, data stay as they were, but free space moved to C Drive, and then run Windows 10 upgrade again, "Windows need more space" message will no show, and the most important is, your data stay intact and system upgraded.