Explanation/Reference:
Explanation: Problem
A datastore in a datastore cluster cannot enter maintenance mode. The Entering Maintenance Mode status remains at 1%.
Cause
One or more disks on the datastore cannot be migrated with Storage vMotion. This condition can occur in the following instances.
■ Storage DRS is disabled on the disk.
■ Storage DRS rules prevent Storage DRS from making migration recommendations for the disk.
Solution
■ If Storage DRS is disabled, enable it or determine why it is disabled. See Storage DRS is Disabled on a Virtual Disk for reasons why Storage DRS might be disabled.
■ If Storage DRS rules are preventing Storage DRS from making migration recommendations, you can remove or disable particular rules.
In the vSphere Client inventory, right-click the datastore cluster and select Edit Settings.

Select Rules and click the rule.

Click Remove.

Click OK.

■ Alternatively, if Storage DRS rules are preventing Storage DRS from making migration recommendations, you can set the Storage DRS advanced option IgnoreAffinityRulesForMaintenance to 1.
In the vSphere Client inventory, right-click the datastore cluster and select Edit Settings.

Select SDRS Automation and click Advanced Options.

Enter 1 in the Value column for IgnoreAffinityRulesForMaintenance.

Click OK.

Reference: https://pubs.vmware.com/vsphere-51/index.jsp?topic=%
2Fcom.vmware.vsphere.troubleshooting.doc%2FGUID-91D4FBBC-0677-44DA-B0F2-
43A3504F8CA0.html