Spotlight not indexing after moving to external SSD boot drive

I have migrated data from the internal Fusion drive to an external SSD (Samsung T5) connect via USB3. It all boots fine and far snappier (I know connection via TB would be faster still).

One issue I do have is that Spotlight will not index this file so a can't search for anything and more importantly can't search within Mail or use Go64. Currently on Mojave and now need to upgrade to Catalina and Adobe's latest application require it.

Go64 just errors with "Spotlight may be disabled on this disk." and tells me to remove it from the Privacy list (which it's not included in)

The drive is connected direct to the iMac and is formatted as APFS and I've tried the obvious re-index the drive in Spotlight (by adding drive to Spotlight>Privacy and then removing it again). In Activity Monitor when I have check mdworker a couple of times over the past few days it is either showing 0% CPU or not showing at all. And Spotlight itself does not show an indexing progress bar.

Possible clue.

If I run "sudo rm -R /.Spotlight-V100" in terminal it returns:

"rm: /.Spotlight-V100: Operation not permitted"

the .Spotlight-V100 folder is owned by my user and is in the staff group, permission is set to 700.

Shall I try and manually delete this folder - it may be corrupt?

iMac 2017 27" 3.4i5 running 10.14.6.