I got a 64 sd card, but the phone does not recognize it, while there is no problem with a 32 card (same make). But the 64 card works in the card device I use with my PC, so is there anything I can do to use it in the phone?
I had a different problem with my sd card in an old phone once. While trying to solve it I read online that a phone may not recognize the sd card if it was not unmounted properly before. In this case, you could try formatting it? (That is, if you didn’t already try that.)
Also I’m guessing your default storage is set to internal?
I’m not an expert by far, but maybe info about your FP(1st/2nd batch & running OS) and the make of the sd card will be helpful in solving this.
Thank you very much, Kris, re-formatting did the trick. I didn’t know how to do it without being able to see the SD, but I stumbled on an error message, that showed the card as faulty and allowed reformatting.
Regards
Catherine
Glad to hear you could make it work! 
Glad to hear this worked. So that other see the full solution for this I’m posting up this link which explains how to format an SD card and which file system to use (which is often what causes these problems)
The other posts about big SD-Cards are closed, so here is my experience:
I inserted a new 64GB SD-Card with copied data from the old and smaller one. After starting, there was a message “abgesicherter Modus” at the bottom of the screen and in the wipe down screen was a message “broken SD-Card” with the only possibility to reformat.
Then I discovered, that the SD-Card was exFAT formatted and the phone don’t support that. So I reformat with the HP-tool to FAT32. Now there was no “abgesicherter Modus” but still “broken SD-Card”. Then I started the formatting with the phone (data was saved before) and voila, it works!
It seems, that there is a bit less capacity, because its shown 57,66 GB wich is about 61.911.953.572 Byte. I’m to lazy now to check the SD-Card again in a SD-Card reader outside the phone.
Have you accounted for the space required by the file system?
I had the same problem with “broken SD-card” even though I bought a brand new one (SanDisk microSDXC) but as soon as I went to settings and erased the data on the SD-card, it started working.
I moved all my music files from the intern memory to the SD-card and plan to do the same according to my (so far) 7 GB of photos (but I will also backup copy these on my computer).