You may find that recovery operations from online backup fail when the target of a restore is a Recovery Storage Group that exists on an External USB storage device. Most commonly External USB storage is implemented as a temporary measure due to lack of physical disk space on the server. Thus, the storage is added, an RSG is configured and linked, and a restore is attempted. However, there are a number of "gotchas" you need to be made aware of to allow these operations to complete successfully. The "general inspiration" for this post was taken from a real case I worked within Customer Support Services.

Full Article: Exchange Server blog