~ update xcode while primary hdd do not have enough space ~

make your hdd with more space
del all the folder in
~/Library/Developer/Xcode/Ios DeviceSupport

Manually download the xcode from
https://developer.apple.com/download/more/
to the external hdd
extract the Xcode.xip

see size of the xcode.app
if the mac cannot contains more extra space, del the xcode from application folder and copy the xcode.app to the primary hdd