Library Cannot Save Changes

Macbook Air mid 2013 with Mac OS High Sierra 10.13.1

Keep getting the error message:

"The storage location for one or more libraries is no longer available. The libraries will be closed so that no work is lost."

Library is saved to an external Seagate HD - OS Extended (journalled, encrypted)

I've checked the storage locations on the library and they all point to the Seagate HD.

I've checked the permissions on the HD and all the FCP folders within it and they are all 'read&write'.

Any help would be most appreciated...

Thanks,