The flaw was discovered on Wednesday as the spacecraft was being readied for the first shuttle launch since Columbia fell to pieces two years ago - a disaster blamed on a chunk of foam that fell off the tank during liftoff and gashed one of the wings.
Nasa spokeswoman Jessica Rye described the flaw as a hairline crack and said that after sending images of it to the tank's manufacturer in Louisiana, the space agency concluded it did not need to make any repairs.
Nasa later said the 4cm crack was high up on the shuttle in a spot where if foam flew off it was not likely to hit the vehicle.
"It's a very, very tiny crack. Very, very narrow ... well within our experience base," Mike Leinbach, shuttle launch director, said.
"It was an acceptable condition for flight, so we rolled on out and we're going to fly with it just as it is."
The space shuttle Columbia crew
were killed in a launch tragedy
Nasa then began moving Discovery from its assembly building to the launchpad after a delay of at least two hours.
Because of Columbia's disintegration over Texas in 2003 - and the deaths of all seven astronauts aboard - the tank has been extensively redesigned for Discovery's flight.
Nasa plans a mid-May liftoff with a launch window from mid-May to June - a period dictated by the position of the international space station, the shuttle's destination.
Discovery was moved on a 5.5 million pound transporter, a huge platform on caterpillar tracks, along a specially built road that is almost as wide as an eight-lane highway.
The 4.2-mile journey to the launchpad took 10 and a half hours since the transporter moved at only 1 mph and encountered a snag at the end.
"It's a great sight to see Discovery rolling out to the launchpad. We know we are getting close"
Discovery mission commander
The move went smoothly until gauges gave off false indications about whether the vehicle was level as it climbed a ramp leading to the launch pad.
The transporter was backed up to level ground so workers could test the gauges and replace a loose data card that appeared to be the problem. The vehicle resumed moving after about a a two-and-a-half hour delay.
"It's a great sight to see Discovery rolling out to the launchpad," shuttle commander Eileen Collins, who will lead the crew on the next mission, said from Mission Control in Houston. "We know we are getting close."
Shuttle programme manager Bill Parsons said he had goose bumps on his arms as he watched the shuttle make its way to the launchpad. "Today was absolutely special," he said.