As @NSDCars5 stated, I mean the opposite.
USB drives (aka thumbdrives, jump drives, flash drives, etc are all the same thing ) have a finite amount of times data can be written to them before they fail.
If you write too much too often then the drive will fail and you will lose everything. This means storing the virtual machines on the flash drive is a TERRIBLE idea.
Having them on an external hard drive ensures that the flash drive is only facilitating the usage of the machines. This means that the drive would last at least ten times longer.
Another tip is that flash drives don't warn before they crash, it is immediate and permanent...It is also very hard to get data off a crashed USB drive...I learned that the hard way years ago.