Time Machine wants to create a new backup

Hi,

I have an Airport Extreme with an external hard drive of 1TB connected to the USB port. I've created 2 partitions on this hard drive for the backups of my iMac and MacBook Pro.

For now both computers have been doing their backup without any problem. This morning after switching on my MacBook Pro I got a message:

Time Machine completed a verification of your backups. To improve reliability, Time Machine must create a new backup for you.

And I have 2 choices: Back Up later and I will get the same message in one hour or Start New Backup and it wil take hours to complete it and maybe delete the old ones.

Time Machine on my iMac is still working fine.

On my MacBook Pro I can enter in Time Machine and see the old backups. I even tried to back up an old email that I just deleted.

I opened Time Machine Preferences, clicked on Select Disk, I selected the disk used to backup the MacBook and clicked on Use Backup Disk.

A window opened where I set my password and clicked on Connect.

But when I click on "Back Up Now" it will start to Look for the Backup disk and after a minute the same window asking me to Backup later or to create a new backup will open.

What can I do to continue to use the same old backup. The disk is not full, I still have 368GB available for this partition.