Nano Server attached to SAN doesn't see new files

NTFS is not a cluster-aware file system. It was never designed to support simultaneous access from different clients.

In order to allow multiple access: configure MPIO, deploy the Failover Cluster feature on both servers, create the new cluster and add your SAN volume to Failover Cluster as CSV (Cluster Shared Volume).

To make this work, you need a NAS control over the drive that allows each of the systems to access the disk as a network drive, not a LUN. Windows knows how to share (or rather, knows a file share is shared space) where it doesn't realize raw diskspace is shared. The real issue here is Windows keeps part (or all) of the directory structure in memory, and if both systems write data, they can both write it into the same sector causing corruption.

Best bet if you need to share this space with two servers is to put it behind some form of a NAS device (or even FreeNas, etc).