A lot of USB 2.5" enclosures are bus-powered no matter if you put an HDD or an SSD in them. Only old/underpowered USB ports may have problems.
Hmm. I've been testing various new notebook drives with "late model" notebook enclosures. They all have required an AC adapter to spin up. Some tap into the FW800 port to get enough to spin up.
Which drives and USB enclosures have you experienced as bus powered on USB only?