I'll include USB keys in this answer.
In a freespace erase eraser will first fill the drive and then erase the space, you would only need to use a 1 pass erase on a USB or SSD drive
For SSD Read this
article and
this - You need to consider the age of the drive and whether you need to wipe data in the MFT (File /directory table)
In summary, its really down to peace of mind, doing a single pass freespace erase once should not wear out the drive