Move any files/folders you want to keep to your main drive.
Run Ccleaner on your main drive.
Start/Run diskmgmt.msc to start Windows Disk Management. Right click your second backup drive partition and Format. Select Quick Format and leave Windows Compression options unticked (it can cause problems sometimes)
Make a fresh Acronis Full backup but find the options for its compression and increase to max/best. Check/Verify the Image after using the relevant Acronis option.