Welcome to Android Central! Wow, I don't think I've ever heard the M8 described as "eh."
How old is the SD card? SD cards don't last forever--they will die at some point, depending on how often they're used (i.e., how many read-write cycles), or if they're defective (SD cards are so mass-produced that defects are relatively common). They can also get corrupted, like for example if you remove the card from the phone without unmounting it first in
Settings>Storage (kind of like clicking Eject before removing a USB drive from your computer).
Go to
Settings>Storage, unmount the card, and remove it. Then insert it into your computer's SD card reader, and see if the computer can read it fine. If so, backup all of that data now. Then run chkdsk to look for any bad sectors.
http://forums.androidcentral.com/amb...d-sd-card.html