Hmmmmm...... it looks horribly like the drive is getting slow on start up, so not spinning up to speed before the processor tries to access it, I would agree that a copy off and put it out to grass could indeed be called for, unless you've added new hardware recently, which could be asking a bit too much of the power supply, and it's this disc that's suffering from low voltage.
Not that likely though, as it's already been at the "master" drive and got the info from that.
You've tried removing and replacing the cables, power and data, to the drive, just in case it's a slightly dodgy connection?
OK, once it's running, it's probably going to behave itself for some time to come, but the risk is there, so your call
, if it's got anything on it that's remotely important, it's not worth the risk