This can happen if you take a Macintosh-formatted card, then initialize it on certain PC configurations, and then bring it back to the Macintosh for use under PC Exchange.

The Mac files and folders should all be gone, (because of the PC initialization) but they're still visible. During the initialization on the PC, the part of the disk which contained the Macintosh directory information wasn't deleted as it should have been. The drive is now in a dangerous and vulnerable state! Some of the files may still be openable (on each respective machine), but there are no guarantees, and the drive will certainly become corrupted if you continue to put more files on it.

If you have a drive in this state, you should immediately copy the files to a different location, such as the computer's internal hard disk. Do this both on the Macintosh and the PC. You may receive error messages, and some of the files may not be copied successfully, because each computer thinks it has full ownership of the same areas of the disk.

So far, this behavior has been seen only when the PC initialization is done with the IBM driver on a ThinkPad. The driver which comes with Ventura Micro software does not have this issue. Other versions of C&SS (Card & Socket Services) running on the ThinkPad are still under investigation.

If you reinitialize the drive in the Macintosh (using the Erase Disk command in the Special menu), it will no longer be recognizable to the PC, and you will be able to use it again safely if you use it only in the Macintosh.

Cards which are formatted on the PC (and never formatted on the Macintosh) can be used successfully going back and forth. The issue only occurs when a Macintosh-format card is re-formatted on a PC.