I had a situation today where my XenServer pool had an old NFS ISO Library attahced while the NFS share itself had been taken down long time ago on that specific IP address. After searching for different solutions I found out that the only working solution without a restart was to set up the exact same NFS share on the same IP address. Only then you can detach the NFS ISO SR.
After you have re-attached the nfs share on the same ip and same path you can detach it from XenCenter or follow these CLI commands:
Get the storage repository uuid:
xe sr-list name-label="Your SR name"
Get the uuid of pbd:
xe pbd-list sr=<UUID of SR>
Unplug all pbd-s by issuing:
xe pbd-unplug uuid=<UUID of PBD>
Forget the SR:
sr-forget uuid=<UUID of SR>
You can also try to use the –force option with any of the commands above.
Managed to remove the NFS SR without a restart.
The defunctioning NFS storage repository was actually preventing any other SR related actions on my pool.