is Time Machine still deleting?
Perhaps you should have a look at the Time Machine Preference panel.
There you see the estimated size of the current backup and can compare it to the free space.
You said that you connected two disks. Is the second disk to be backed up as well? Maybe this second disk has not been excluded from the current backup (in the options settings) and now Time machine wants to back up more than you wanted and thus, needs more space.