The drive, or perhaps more precisely, the "slot" the drive is assigned to, will be disabled by Unraid when a write to the drive fails. This could be an actual write, or it could possibly be a write-back from the parity calculation when a read to it fails. After it is disabled, Unraid will emulate it from the parity calculation until it is rebuilt.
Unraid is NOT RAID, each drive isn't required to allow access to the data on the array. Each disk is an independent filesystem. Until a read or write is asked for on that drive, Unraid will not notice since it will not try to access the disk. Possibly the disk was even already spun down when you pulled it, since Unraid will spin down unused disks after a (configurable) time.
A disabled disk is shown with a red X next to it, and if you have Notifications setup correctly, you will also get an alert. You should setup Notifications to alert you immediately by email or other agent as soon as a problem is detected.
If you only have single parity, then when a disk is disabled, you no longer have any redundancy, though the data for that disk can still be accessed for read or write using the parity calculation. Dual parity allows 2 disabled disks before redundancy is lost.