The disk simply dropping out without chocking the mirror handler is strange.
When NASLite-2 boots, all storage devices get detected rendered and recorded to a device index which is used during system operation. The mirror event runs a number of checks that must pass prior to the event actually happening.
Especially in your case, where the mirror is local, the mirror will not execute unless the source drive, addressed by device node, passes SMART. If it fails, the mirror dies with error. If the drive is off-line then it's device node will produce a SMART failure since there is no firmware to respond to the query. In turn, the blank source folder can only happen if the drive has failed and was taken off-line by the driver, leaving an empty mount-point folder. In other words, if the device node passes SMART, the drive should be online.
So, how can a drive drop offline while still passing SMART?!? - This is definitely strange.
In order for this to happen, the drive should fail temporarily, get dropped from the chain leaving an empty mount point, come back online (this will definitely produce errors in the syslog) and not get remounted. I've never seen that occur, but who knows...
I'm shooting in the dark here but are you using any IDE to SATA or SATA to IDE converters, any unique adapters, controllers, anything strange? What is the exact setup you have?