If an external hard drive is connected to the router and its partition has no File System, the router may not be able to read it. It could still detect and mount that partition, but its read and write capabilities may be compromised by the lack of a File System. However, if you map to it from a local host, a connection between the hard drive and that client will be created. That client can then read or write to the hard drive if it supports partitions with no File System.
NOTE: Linksys routers support FAT, FAT32, NTFS and HFS+ and ext2/3/4 file systems.