Upgraded to a new s8 + --$900 device from my s7 edge just to have it produced with faulty components. It worked flawlessly for the first 3 days. After mving some content to SD card phone still performed well. Now phone does NOT even recognize that it has an SD card in it. Yes it is in correctly because my sim card is right under it and it is being read correctly. The phone has NOT even mounted or un-mounted. But the apps that are on my SD card are STILL usable, although all my music and pictures are gone. I feel like I paid for a HEADACHE. Never again Samsung. Next time I will WAIT 3 months after release. Never thought I'd have these BASIC issues with a FLAGSHIP product. Could have bought a LOW-END Samsung and got the EXACT same quality without the fancy design.
I have a similar issue - brought my SD from my S5 over - all was great then suddenly taking a photo it said it couldn't be saved on the card. I can READ all of the photos etc... on the card, but I cannot WRITE to the card - meaning moving photos around etc... I unmounted, took out and what not and still no joy. It will, however, work ONE TIME after a restart. So, if I do a restart, it will do one write and then the next fails. (trying to move the photos that auto saved to device to the card)
Given that it DOES work (at least once per restart), I can't imagine this is a pure hardware failure... I can consider relocating everything, reformating and starting over... but given that I am not alone here....
My problem stopped literally for a day then restarted as speculated a d it was drastically worse for a short time after. Hit 50 unmounts by lunch.
Samsung, you gonna start sending everyone SD cards when your phone burns them out? It'd be one thing if they acknowledged the issue and said they were working on a fix. Nope!
I've gone through online chat, FB, phone customer service and they give no answers, no acknowledgement, refuse to read this forum topic, and offer no sign resolution.