A hard drive that connects via Ethernet is not dumb, it has some level of NAS functionality. As long as it supports the antique SMB v1 protocol (v2 or v3 WILL NOT WORK) it will work for Sonos.

A dumb hard drive that connects to your router by USB will also work as long as the router supports SMB v1.

NAS simply means network attached storage, could be a Raspberry Pi Zero-W for around $20 on sale to a $10,000,000 IBM storage system, What you get at the different price points is either more storage or more features, all are NAS.

For Sonos use you don’t need much storage (due to the 65K track limit) or any features beyond SMB v1.

Thanks very much for this helpful info.  That cleared things up for me.  

I discovered today that my router’s (Amplifi) USB port is “nonfunctional with no future development plans” per manufacturer, so I am looking at other options.  Pi with a spare HD is on my list of options. 

I did see the 65k track limit. Understanding the variability between track sizes, is there a typical HD size that most Sonos users get?  

I have 7000 tracks from 470 CDs (with album art) stored in 130 Gb of file space hooked to an old Pi 3b.

I picked up a discarded 256 SSD for almost nothing and a $10 USB to SATA cable and it happily runs several music streams, I only tried four independent streams but it was loafing.

Fire it up, enable SMB (Samba there) and make the tweak here to get SMB v1.

https://stan-miller.livejournal.com/650.html

The tweak likely will work on any SMB server that allows v1 to be enabled.

As with most of these things - it depends :-)

Sonos itself has limits - no more than 65k tracks and mustn’t exceed the amount of store allocated (difficult to calculate - you have to ask Sonos if you really want to know what yours is). Consequently, even though I have under 40k tracks, Sonos can’t handle the amount of store - so I’m at the limit.

Disk space is another issue, depending on what format you’re ripping the files to. I use flac, so my 40k tracks take up 800Gb of disk. If you used 320k MP3, it would be considerably less, and lower bitrates even more so.

So if you have only contemporary music (which has short titles and tags, thereby allowing you to hit the track limit on Sonos) and use flac, it’s possible that you might need more than 1TB. However,  most people seem to stay under the 1TB mark, from comments I’ve seen around here.

How many tracks and what type/format are they?