In a post to the USENET group alt.video.ptv.tivo (Google Groups) user Douglas Bullard shared a cautionary tale. Apparently Western Digital changed something in the way they prepare the My DVR Expander eSATA drives and this causes the TiVo HD to not recognize it as an approved drive. (Since the Series3 works with most eSATA drives I’d expect it to still work with the S3.) It seems that Douglas was the first (un)lucky person to discover this change in the wild, so the extent of the issue is currently unknown.
TiVo and Western Digital are now aware of the issue and are investigating, so there will probably be a fix for the issue forthcoming. TiVo is replacing Douglas’s drive with one that will work with the TiVo HD. So if you buy a WD My DVR Expander and have trouble getting it to work with your TiVo HD you might want to contact TiVo and/or WD. If this impacts a batch of drives just returning to a store for another may not solve the issue, as other drives on the shelf are likely to be from the same batch.