What do you mean by "transfer"?
Are you using Windows Explorer to copy and paste the files, or are you using some kind of program to do it?
What kind of error do you get when it stops copying?
As mentioned by Sintares
, there is a limitation on the number of entries in the root, depending on the file system (it's probably FAT16). This is normally 512 entries. But, if you're using long filenames with Windows (anything other than an 8 character filename + 3 character extension), it's going to use more of the entries.
That's one reason camera manufacturers don't use long filenames (better compatilbility with more file systems and utilities). So, they typically use filenames that look something like DSC_1234.jpg or pict1234.jpg (8 characters + a 3 character extension).
If that's your issue, as already suggested, place the files in folders versus in the root directory if your device can find them there. If not, and if your picture frame supports FAT32, you could reformat the cards that way using Windows (select FAT32 from the drop down file system type list). Windows usually formats a removable memory card this way by default. But, make sure by looking at the file system type when you use Format.
But, be advised that many camera models are *not
* FAT32 compliant (the cards won't work if formatted that way in a number of cameras). I can't tell you the number of posts I've seen from users asking asking why their card won't work in their camera anymore after they formatted it with Windows (quite a few). lol
So, depending on the camera model, you'd need to reformat the cards using the file system type of FAT from the drop down file system type list with Windows if you wanted to use it again in a camera. It's a good idea to reformat the cards using the camera's menu choice for format before using one again if you do that.
Also, it's a good idea to use the "Safely Remove" icon in your system tray if one exists before removing any card from a reader that you write to with Windows (to prevent corruption, because some drivers will cache writes to media by default).
The easiest way is going to be the way already suggested earlier in this thread. Copy the files to folders versus the root directory on the card if your picture frame can find them that way.