USB/SD is not the right choice going forward! Why? The volume of reads/writes to and from the OS-DATA partition continues to increase with every release, which means that the lower grade devices will simply wear out faster.
Options for future versions of ESXi ( after 7.x)
Options for 7.x
What are the fixes that will help with SD/USB issues that customers are seeing?
(Please look for USB device and Storage stack issues fixed with this release)