The flaw was discovered 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.
But Nasa later said the crack was no reason for concern.
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.
She said Nasa would press ahead with the move from the assembly building to the launch pad, 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.
"It doesn't sound like it's a major issue, but because the foam is a sensitive issue we want to make sure we're in a safe and right configuration"
"It doesn't sound like it's a major issue, but because the foam is a sensitive issue, we want to make sure we're in a safe and right configuration," Rye said earlier on Wednesday, when the crack was discovered.
A delay in the moving of Discovery to the launch pad could upset Nasa's plans for a mid-May liftoff.
The launch window runs from mid-May to June - a period dictated by the position of the international space station, the shuttle's destination.