Maybe not the answer to your question, but you shouldn't place snapshots on slower storage than the base virtual disks. Snapshots in VMware products are used in chains, i.e. each chain link is in use. The latest one in R/W access, and its parents in R/O access (see e.g. Understanding VM snapshots in ESXi / ESX (1015180) | VMware KB ).
To say it in other words: "A chain is only as strong as its weakest link."
André