Drive bypass events are situations which cause the ESH to bypass a drive port thus making it inaccessible by the host and isolating the drive from the loop. There are primarily two different kinds of drive Bypass Events:
Threshold Bypasses - Situations where the ESH detects that a certain kind of error is occurring for a specific period of time and the determination is made to bypass the port because of accumulated errors over time or over the amount of data flowing through the loop.
Policy Bypasses - Situations where the ESH detects a critical problem and bypasses a drive in order to maintain the integrity of the loop. This might be a "situational" problem which later clears, but once a drive is bypassed, it will remain bypassed until manually cleared by removing and reseating the drive, or by power cycling of the shelf.
Self Bypasses - The hard drive electronics determine that the internal circuitry cannot function properly anymore. Thus the drive itself calls to be removed from the loop.