go to: C:\ProgramData\Microsoft\Search\Data\Applications\Windows and look for the file Windows.edb and check the size of this file. This is a database file associated with Windows Search feature. If it's too big, sometimes it can grow up to many GB's then it might cause high disk usage when Windows Search trying to rebuild the Search index. You can delete this file but first you need to disable the Windows Search service then re-enable after deletion.
Run Task Manager->Startup and disable everything then reboot the PC to see if it fixes the problem then re-enable one by one, reboot until the problem occurs again, then that's the culprit.
Run Task Manager->Details, then right click on the Title->Select Columns and put a check on I/O Reads, I/O Writes, I/O read bytes, I/O write bytes then click OK. Click on the Title to sort by I/O Read, I/O Write to find out which process utilizing the Disk I/O most and take appropriate action.
The 60% should be ok, but the 75% might be starting to be a problem. I would check both partitions for fragmentation. If fragmentation checks out OK, then this is probably not the issue.
If you right click on the disk in Windows Explorer, then Properties, Tools, you should have two options. One to Check the disk and one to Optimize. I would do the Check first and Optimize second. The Optimize should tell you what the fragmentation is.