I think we are talking about different problems in this thread.
1. The only app that can read and write to a SecureAccess vault is SecureAccess. If you want to update a file in the vault you must use SecureAccess to extract the file to a writable drive, usually the C: drive, update it with whatever app created the file, then use SecureAccess to write it back to the vault.
2. It is possible to read and write files to the USB drive outside of the SecureAccess app. ie use the USB drive like a normal USB drive. Windows Explorer can be used as can any other app that reads and writes to USB drives. The USB files won’t be secure but not all files need to be secure.
3. Some security apps may prevent writing to USB drives and some may even prevent the drive from being detected on the system. This is usually seen in businesses and other controlled environments.
4. Large files can sometimes not be written to USB drives if the files are larger than 4 GB and the USB drive is formated as FAT32 which is the default for USB drives 128 GB and smaller.
5. Some SanDisk USB drives have a manufacturing defect which triggers the drive to go into read only mode in certain situations. There is no way to repair a drive that encounters this problem. It must be returned to SanDisk for a free replacement.