Sprint 6.0.1 OTA is out

This was my suspicion as well. Too bad, that site can3gxw shared looked legit

When I did a search for the model number, it gave me the same thing that can3gxw had, so had to go click by click in the support section to get to the information.
 
Wait a sec...that site seems wildly out of date, am I wrong?

SM-GM900A (AT&T) should be up to at least lollipop, am I wrong?

Since Samsung do not release AT&T or Verizon firmware, it is up to these carriers whether or not they supply details to SamMobile for download. Verizon do... AT&T do not. Hence, only the original S5, AT&T firmware release is showing in SamMobile. IMHO, very mean spirited of AT&T as downloading and flashing the current stock firmware can, in some cases, solve a lot of issues.

As to the AT&T site not showing any updates, I cannot comment, save to say that they may be winding down their support for the S5, G900A, model although I would anticipate at least one Marshmallow release from them.
 
Be very careful with this. I"ve found that, when running stock 6.0.1, whether rooted or not, my SD cards have become unusable.

I'll be opening a new thread about this and will provide the link here shortly.
 
To be clear, it's *accessing* any card that I insert in the phone that's the issue. The actual card itself is fine and works properly in other devices.