The computer can not read the disk you have connected.

Is the external Toshiba drive a 2.5" drive, or a 3.5" drive?
Does it have a port on the back for an external power supply? (even though it may not have come packaged with a power supply)

Without enough USB bus power, a drive might fail to mount properly.
It might even sound like it's "spinning up", but then it STILL won't mount.

It's possible that the "working macbook" is supplying slightly more "USB bus power" than the MacBook Pro (the one with which the drive won't mount).

Does the MacBook Pro that the drive WON'T mount on, have any other USB devices connected (that might limit available power to the drive)?

Final thought:
Sometimes, no matter what you try, a particular drive in a particular enclosure just won't mount. Best to try another drive, enclosure, etc.