Disk Management problem; " The service cannot be started,"

Open a command prompt as administrator and run:

sc config defragsvc start= demand

enables you to start the defragsvc, beware if you have a schedule in effect.

sc start defragsvc

starts the defrag service so disk management can figure out how to reallocate files when you shrink the volume.

I've seen McAfee AV cause a situation where disks cannot be managed. I never figured out why but uninstalling resolved the issue.

I've also seen this issue when the Disk Defragmenter is disabled (for SSD) and turning it on temporarily allowed disk to be managed.

Go to the start button, type "Services", click it (should have an icon of two gears), scroll down to "Disk Defragmenter". Change the startup type if you want to, and make sure the "Service status" is "Started". If not, make it so.

I found that I also couldn't open Disk Management, as well as diskpart.

Open "Services", and scroll down to "Virtual Disk". If this is disabled, enable it with startup = manual. After I changed this, I was able to open Disk Management and diskpart.

This may work in conjunction with the Disk Defragmenter service, or this change may be all you need.