Every day or two my SD card unmounts. The card worked just fine in my old phone for a few years but since being in my EVO Shift, it has been nothing but trouble. I have taken it out and formated the card and that fixed the issue for a few days, but then when I turned off the phone, it unmounted again. Now I can't get it to remount at all. I've taken the card out, put the card back in, tapped on it to remount.. nothing works. 

I am willing to buy a new SD card, but from a little research online, it might be the phone?

It's an 8 gig SanDisk HC card. 

When I went to delete all the content off it last time, there was a corrupt sector on it, but it seemed to repair the sector. The scan after the formatting suggested that it was fine and ready to go. 

Any help is greatly appreciated. I really want to be able to use my SD card. (And the extended storage of my phone)

This just started happening to me too. I have the original factory SD card and it's remained untouched in my phone since last December. Today it randomly dismounted. I pulled it out, reinserted it and booted up my phone and all was fine. For a few hours that is, then it happened again. This time I have been completely unable to get it to remount. WTF is going on?????

If there is an indication that there is a corrupted sector, that's probably due to a defective SD card. However, do make sure that if you're charging the device to a PC to make sure that it is set to 'charge only' and not 'disk drive' or something like that. Those options will unmount SD card access from the phone and give it only to your PC.