In Windows 10 x64, go to the Device Manager, find the external drive under Disk Drives. Right-click the drive and go to Properties. Go to Policies, select Better Performance, select Enable write-caching on the device, and select Turn off Windows write-cache buffer flushing on the device.

My external added to my Storage Space without issue as soon as those options were selected. Prior to doing that it kept saying Cannot add the drive.

In Windows 10 x64:

1. go to the Device Manager
2. find the external drive under Disk Drives
3. right-click the drive and go to Properties
4. Go to Policies
5. select Better Performance
6. select Enable write-caching on the device
7. select Turn off Windows write-cache buffer flushing on the device

My external added to my Storage Space without issue as soon as those options were selected. Prior to doing that it kept saying Cannot add the drive.