Having same issue with Kingston 64GB SDXC, even returned tablet to store purchased from because I thought (after all the TS) that it might be that.  I have run all the updates for the tablet and installed in the order prescribed by Dell, reformatted card in the tablet to exFAT.  tablet will recognize card for a few minutes but once it goes to sleep or switch to tiles and switch back tablet throws message about inserting a card into the slow.  This happens with 'ALL' cards i have tried in the tablet (SDHC, SDXC, 32GB, 16GB, 2GB) yet all these cards work just fine in other hardware to include other Dell laptops.

If you get a response please post....have tried DM via twitter to Dell Cares with continued no resolution.  I am wondering if this is an issue with Dell hardware and MS Windows 8.1 and MS proprietary exFAT format on the cards. 

As I wrote yesterday I was having the same issue.  I reformatted my card from the exFAT format that it came with (and that Microsoft says to use especially with Windows 8.1) to NTSF and its been nearly 24 hours with absolutely no issues and SD card still recognized by Venue 8 Pro.  I have been able to read/write to/from the card with no issues as well.  I have also been able to get the native photo app in 8.1 tiles to recognize the pictures on the SD cards.  

Hope that helps.

I have 2 64GB microSDs formatted in ExFAT; both mount quickly & reliably. If a card takes a l-o-n-g time to mount, set its Custom properties to "Documents" for entire contents.

SDXC card performance & reliability will suffer if formatted to anything besides ExFAT; use NTFS only when you need to create a WIN boot volume...

If your card isn't even being detected, then the slot may be turned-off in BIOS or driver; apart from that, blow out the card slot with compressed air spray and insert/eject a card several times to ensure good connection for all pins; a micro SD card from your phone or camera should also be detected if the port is working...

Hope there's something in all this that helps; you may also consult with Dell Mobility Support — 24/7 in the US, at 800 308-3355.

I contacted Dell Mobility support multiple times (via phone, twitter through DM and posted to these forums) and the only response I got from the contact center was that I needed to put the tablet in a box and send it to a depot.  

The response I got through DM was the same response I got from contact center until final plea for help when they suggested I reformat card as FAT32.

When I stated that was not an option and that (for the third time) I had reformatted as exFAT they responded this way "Please reformat the card to NTFS as Ex FAT does not support Compression or Alternate Streams and will not mirror system records. ^GS".  

If exFAT works that is great and I will try again but after the third reformatting, the card being recognized for a few minutes and then not being recognized at all (same issue on two different tablets) makes me wonder if there is some sort of Dell driver issue with MS WIN8.1 or a problem with MS 8.1 and the exFAT format in general.

Not trying to rant just posing some things to think about.

Accidentally found out (still Fat32) that If I popped the SD card out then in again the system picked it up.  This is a solid problem.  Power up with SD card in, nothing.  Pop it out and in and it is picked up.   Got to be a firmware/software bug.  Will try NTFS next.

I'm having the same problem and I already formatted to NTFS.  I'm running A04 BIOS and the Dell Venue 8Pro will periodically stop recognizing the SD card.  I discovered that popping the card out and back in with the system on will fix the problem until it happens again.  The last time it happened I turned the system off, popped the card out, and then powered it back on and it still didn't recognize it.  The system has to be ON when you do this in order for it to start recognizing the card again.  For me this is a huge problem because I have the 32GB tablet and have moved several critical files/programs to the SD Card such as User directories and Office Software.  Really don't want to send the tablet back because I like the device but I must have a reliable SD Card connection for the tablet to be useful.

HELP!!

I started having the same issues all over again about three days after the final post I created above regarding the NTFS.  The only way I have found to work around it is exactly what you described (take card out while system is on because rebooting does not solve the issue either).  I am not sure if anyone is watching this forum from Dell support but I am going to tweet them again with this address so they can see there are multiple folks with this issue.  This is the second Venue 8 Pro and I am having the exact same issues as with the first one.

A wild shot in the dark here, but did you by any chance enable DDR50 support in the UEFI/BIOS? I know others have had problems after doing that, even with UHS-I cards that should support it. With it left disabled, I've had absolutely no issues with my 64GB SanDisk Ultra.

Thanks for the suggestion ME2OLIVE.  I checked the UEFI/BIOS and it is disabled.  Also tried reformatting the card multiple times with NTFS and with other formats but problem occurs. 

This seems like either an issue with the SD card slot or the OS itself.  I am unable to narrow the issue down further.  Any ideas or thoughts are most welcome.