My library has always been on a NAS which works well. I decided to try adding a share from an external drive on my Windows PC. I used a thumb drive with one album on it with the folder structure “H /My Music /Classical/Berlioz/Symphonie Fantatique. Then in the My Music folder Properties I put Sonos in the share list. Using the Sonos PC app I added the H: My Music folder by browsing for it using the option “Add music from an external drive attached to your computer” (paraphrased). After it indexed, which took forever, I found that it indexed the My Music folder on the PC, not the My Music folder on the H: external drive.
The path of the share was H:\My Music although the slashes were back slashes as opposed to forward slashes as used for the NAS share. So why would Sonos not read the path literally and instead index the My Music folder on the PC? My NAS share + the My Music PC share was too much for the index process. If the index had only taken the single album on H: drive the index would have completed without the “oops too much stuff” error.
After the index I could see H: My Music in the folder view. I it had a lot of music in it rather than the 1 Berlioz album.