If your (non-fake) cards repeatedly get corrupted in your PC, there is probably something wrong with the reader. I was having the same problem. Then I got a new laptop last year and all my SD corruption problems suddenly went away. Way back when I got that laptop, 16GB was a huge SD card. Maybe the reader didn't even support larger cards....
It would take days to transfer 200+ GB with ftpd. I never use if for anything bigger than save backups and homebrew installs.
Use godmode9 to format SD cards. "SD Formatter" (Windows), "Disk Utility" (Mac), "Disks" (Ubuntu) have all been over-complicated, broken shite for me. gm9-formatted cards seem to work more reliably (and faster) in all my handhelds.
The biggest cards I use now are 64GB, originally formatted exfat, reformatted FAT32 (AUTO cluster size) with gm9.
Actually with this new card I used a different PC as suggested in an earlier post. Still, same error. Flash the 128gb and place the files on the 128, both PC's, nothing happens. Nothing's wrong. Even put the SD (128) into my SD card reader, windows read it as raw, popped it out and back into the system, no issue.
So I think 256 is just too big. It seems to save everything fine, all of the games load, all of the files save, even backed up FROM the 256 drop into the 128 and nothing was wrong. All was the same.
So I'm just seeing an error with the 256 and themes at the moment. It takes the system a bit of time to boot up, so during shut down it doesn't seem to remember to look for the home menu file that it creates
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Have you tried
Checkpoint to transfer game saves between cards? If your SD cards are tied to two different NNIDs, your saves can't be copied over using only a computer.
For themes, do you use
Anemone3DS to set them in place? The 3DS Theme Menu will not recognize themes you did not purchase on eShop.
No both SD are tied to the same nnid, I'm just curious if I get another 128 that seems to take fine and transfer to the new system that both cards will still work, or if it's just the target card. (My main)
Im pure digital, so everything I have is on those cards. Not sure where the saves are.
Also like I said I bought the card to use hax, so everything is have is legit purchases. Just multiple SD cards.
I even formatted the fresh SD on a brand new rig, put it in the system (the home menu was still present, folders and such) and just downloaded a theme. Turned on the theme, turned off the system, reboots with no theme. All legit purchases.
So I reformatted from a different PC (I live at my business so I've got a gaming rig, an old rig I use for network and internet, and my business PC's. ) Put it into a different system with a "test" nnid I intended to try out hacking on first, created theme data, chose the black theme, rebooted, loaded without a theme.
So it just seems this size of card loads with no theme. I can select the theme but I cannot actually get the theme to "stick"
Anyway waiting on the other poster to try out the old SD and see if the same issue persists. Everything works but themes. Corrupted smash, lbx, and Pokemon rble world, unsure why world corrupted, just sort of happened. I got a refund but I had like 11k diamonds.
It is good to know smash saves don't actually have the ability to back up
So soon as I hear if the 256 held themes and all went well then something is wrong on my end on all of my PC's, or maybe it's just micros. Idk. I just worry of corrupt data from the 256 or transferring the data to my new system and it corrupting. Prob safer to do with the 128 and just get a second 128