Ok, let's think about this logically for a moment...
In the default state, MM is set to notify you when it sees that the SD card is mounted.
Rebooting your phone would cause the SD card to be dismounted as it turns off, and mounts the SD card as the OS completes loading.
Both actions trigger events in the OS, so (in the default state) you get the SD card mount notification. Simples.
Repeated notifications are an indication that the SD card is being dismounted and remounted - no argument there.
But let's put aside for a moment whether it's a physical issue, and OS issue, or a format issue, or something else...
For those of you who have got replacement phone, and no longer get any notifications, even after a reboot, they have obviously either suppressed the notification completely, or set the default state of the notification to off.
Has the problem been fixed? The answer must be a resounding NO if you can prove before getting the replacement and after getting the replacement, that your SD card was indeed actually dismounting and mounting.
As mentioned by JindoJake and others, running the "SD Card Monitor" app should be used as part of the investigation to determine if in fact the SD card is dismounting and mounting.
Other more in-depth tools might be required, as well as trawling any system logs.
Once you have enough information, then - and only then - can you truly say what the issue is and determine how to fix it.
Changing settings here and there to "fix" the problem is like a janitor doing delicate brain surgery with a long, sharp stick while wearing a blindfold... how can you ever know that you've "fixed" it if you don't know what the cause is??
So, in summary, sending your phone back is futile, factory-resetting your phone is futile, reformatting your SD card is futile, if you don't understand the cause.