Having just been through this, I can advise...
I bought an LG Blu-Ray drive recently, and an external cabinet for it which did USB and eSATA.
Since i'd not got an eSATA cable at the time, I plugged it in using USB. It then performed fine as a CD and DVD player, but NOT as a Blu-Ray player. In fact it would not perform any of the Blu-Ray tasks, and when putting in a BR disc it refused to recognise it at all.
I then went and bought an eSATA cable, and attached it to the same laptop (which had an eSATA port as well anyway). Robert was then your mother's brother, as the drive suddenly was properly identified and worked without a glitch.
Hence I would be aware that you may not necessarily get an external Blu-Ray drive that works properly over USB.
Mike.