The first high-pressure gas tank is lifted out of the cargo bay by Canadarm2 as spacewalker Mike Gernhardt rides the shuttle's robot arm toward the airlock. Image: NASA TV/Spaceflight Now

After a computer scare reminiscent of problems that crippled the international space station in April, two spacewalkers successfully attached three high-pressure gas tanks to the lab's new airlock today, manually wrestling the 1,200-pound canisters into place.

Michael Gernhardt and James Reilly originally planned to install just two tanks, one containing 3,000-psi oxygen and the other nitrogen. But installation of the first two tanks went so smoothly, flight directors cleared the spacewalkers to install a second oxygen tank.

A second nitrogen tank will be installed during a third and final spacewalk overnight Friday when Gernhardt and Reilly will use the new airlock for the first time and complete its outfitting.

"I'd just like to offer our congratulations on a great day," astronaut Daniel Burbank radioed the astronauts after today 's excursion ended. "We got an awful lot done today, we got about an EVA-and-a-half worth of work done and we actually learned a few things during the process as we have most of the days during this mission. But great job on everybody's part."

The only unsettling note came Tuesday afternoon, about five hours before the second spacewalk began, when one of the space station's three command-and-control computers suffered a hard drive failure. Similar malfunctions in April, later traced to hardware failures, crippled the outpost in April.

This time around, engineers were able to reset the software controlling C&C-3's hard drive and the computer system was recovered. But Hill said engineers do not know what triggered the initial hangup and he said there are no guarantees additional problems won't crop up down the road.

=Hill said NASA's procedures for dealing with such glitches were modified in the wake of the April failures and that flight controllers are better prepared to handle such problems now. In the meantime, NASA is accelerating development of solid-state memory devices to replace the spinning hard drives.

"These are the types of days that haaving people in space allow us to pull off a mission when it certainly feels like the odds are stacked against us," Hill said.

"The error message we got essentially said the hard drive was not functioning, so we did a dump of the computer and Honeywell went off and looked at the data for us and two hours later, what we saw was that the hard drive was still spinning but the computer was not able to talk to it. We sent a command to reset the firmware controller to the hard drive ... and then we kept right on pressing."

Today's spacewalk began at 11:04 p.m. Tuesday and ended at 5:33 a.m. today for a duration of six hours and 29 minutes. It was the 106th spacewalk by U.S. astronauts, the 66th in shuttle history and the 23rd devoted to space station assembly.

Station astronaut Susan Helms, operating the lab's Canadarm2 space crane, plucked the first oxygen tank from its perch in Atlantis' payload bay and positioned it within a few inches of its attachment point on the airlock.

Gernhardt and Reilly, anchored in foot restraints on either side, took the canister from Helms and muscled it into place using a guidepost to maintain stability until latches could be engaged. Gernhardt then opened a valve to permit the tank to pressurize the airlock's oxygen lines.

But to the surprise of flight controllers, the lines stayed at 900 psi and did not jump up to 3,000 psi as expected. After a bit of troubleshooting, the spacewalkers began work to install the first nitrogen tank while engineers on the ground studied the oxygen tank problem.

As it turned out, the problem was inside the airlock where a valve was mis-labeled. When the proper valve was opened by the station astronauts, the oxygen system pressurized as expected.

The only other problem was a balky quick-disconnect fitting needed to connect the nitrogen tank to the airlock's plumbing. The fitting initially failed to engage properly, but after warming up in the light of the rising sun, it slipped into place without any trouble.

"Good work, good support by the whole team today," space station astronaut James Voss radioed flight controllers when the excursion concluded. "A lot of work got done. We got three fourths of the tanks done, that's really good."

"Yeah, everybody down here is ecstatic, this is a terrific day," replied Burbank in space station mission control. "I know we threw some curve balls at you by pushing on for that third tank. But again, we think it went terrifically and we really appreciate all your help and flexibility."

The astronauts plan to complete the airlock's outfitting and activation over the next two days before staging a third and final spacewalk overnight Friday.