Hi Col.
Verify that you have space on the index volume.
Verify no other ' thing' is locking the volume/folders (think AV-scans, backup?)
Verify EV is not in backup mode for that indexlocation
If necessary, run a trace from within the console (select ev-server with issue, click Tools, Advanced (refresh if necessary)
you should get a ' trace' container underneath services and tasks.
Select new, create trace for Search and Index issues.
Check to see if something strange shows there.
check also taskmanager for index proces etc.
Be patient ;)
Regards. Gertjan