play music stored on solid state drive

In theory, yes, but you’ve not given us much info. 

Which version of Sonos are you running? S1 or S2?

Which ASUS router and which hard drive? 

Are you able to see the hard drive from a computer on the same network? 

Have 4 x S1 plays and a beam.

It's an Asus ax53 router with a Kingston SSD attached by USB 3.

It's not attached to an computer.

With the previous Yamaha musicast system it just picked up the drive via wifi.

There’s no such thing as an “S1 play” so I guess you mean Play:1. 

My question was whether the system as a whole was running S1 software or S2 software. Sonos S1 requires that the shared network storage -- in this case your ASUS router -- supports SMBv1, which is widely deprecated. Sonos S2 is able to work with SMBv1, SMBv2 or SMBv3.

Here is more on adding shared folders to Sonos: https://support.sonos.com/en-gb/article/use-a-nas-drive-with-sonos

A quick google suggests that Yamaha uses DLNA/UPnP to stream from a media server. Sonos doesn’t, it requires SMB to access the shared folders directly. Hence my query about whether those shared folders were also visible on the network from another device, such as a computer. 

Thanks but I have no idea how to do this.

Thanks for your help, I'll try this later but I'm not great at this sort of thing.

It might be a sharing issue

Have you tried plugging it into your computer or other device and sharing it from there?

Steve

IIRC the Samba service is not enabled by default on ASUS boxes.