In what circumstances will a surety be able to set aside a defendant's bail?

In Durbin, the surety moved to exonerate bail because the defendant was being held in custody in Tennessee. At the time, section 1305 provided the trial court with discretion to discharge a forfeiture of a bail bond if the defendant was unable appear in court because, among other things, he was being detained by civil authorities. After a remand in which it was ordered to exercise such discretion, the trial court exercised its discretion and denied the motions by the criminal defendant. The ensuing motions by Durbin and his sureties to set aside the forfeitures of bail "were based upon Penal Code section 1305, which throughout 1961 provided, in part, that the court 'may' discharge the forfeiture upon such terms as may be just if within 90 days after entry in the minutes of the failure to appear it is shown to the satisfaction of the court 'that the defendant is dead or is physically unable by reason of illness or insanity, or by reason of detention by civil or military authorities, to appear in court at any time during said 90 days.' In September of 1963, between the time the appellate court issued its remittitur reversing the orders denying relief and the time the trial court again denied the motions to set aside the forfeitures, section 1305 was amended by replacing 'may' with 'shall' and thereby made mandatory the granting of relief upon such terms as may be just if a timely motion is made and the inability specified by the statute is shown. (Stats. 1963, ch. 2014, p. 4113, 1.)" (People v. Durbin, supra, 64 Cal.2d at p. 476.)