They are both 1TB, 7200RPM drives and are both connected to external power.
Eeeek, dont do that anymore. 2.5" dont need power, and daisy chaining them BOTH thru thunderbolt, your Air is likely kicking one because youre mostly likely sending feedback THRU the firewire.
Some inventive folks unknowingly have powered (3.5" need power, 2.5" DONT) 2.5" thru USB and found out the hard way they deep fried circuitry.
Likely. Dont keep 'poking the dragon' however as per damage.
You have a voltmeter?
The product specs for Glyph portagig 50, dont list specifics. Contact mfg. on the issue before continuing.