How to install OS X 10.11.6 on an external hard drive?
How to install OS X 10.11.6 on an external hard drive?
External drive is a WD Passort portable 2TB for Mac connected by USB 3.0 to Mac-Mini.
Neil's answer was too simplistic. He failed to say that even if one buys a WD Passport "For Mac" drive which is formatted for Mac, one still must GUID Partition the drive. But there is no guarantee that El Capitan installation will be accepted. The WD drive I wish to purchase is model WDBP6A0020BBK-WESN. That being the case I finally found in searching the WD site the following, which may be of some interest:
List of Mac-bootable WD external drives and how to format them to install an operation system
/support.wdc.com/knowledgebase/answer.aspx?ID=1787&h=3 NO HELP-the model of interest id not cited.
and
How to Partition and Format a WD Drive on Windows and macOS
How to Partition and Format a WD Drive on Windows and macOS | WD Support
The model I cited is not listed as bootable and not listed as bootable
