Files become unreadable after transfer mainly due to corruption or format incompatibility. Corruption happens when data bits change unexpectedly during the transfer process, making the file unreadable by any application. This differs from simply lacking the correct software, where the file itself is intact but you can't open it. Common causes include incomplete transfers interrupted by network issues, unstable connections (like spotty Wi-Fi), physical storage media errors, or rare bugs in the transfer software itself.

For instance, a large image file transferred via FTP might only partially complete if the connection drops, resulting in only the top half displaying. Similarly, transferring a spreadsheet across different versions of Microsoft Excel could corrupt specific macros or formatting rules, rendering parts of the document inaccessible within the newer or older application, even though the core file transfer seemed successful. Cloud storage transfers are also susceptible.

This highlights critical limitations: transferring data doesn't guarantee its integrity or perfect compatibility. Recovery is often impossible without backups. While verification steps like checksums exist in some protocols to confirm accuracy, their absence or failure in others leaves files vulnerable. Preventing this involves using reliable connections, robust transfer protocols (SFTP over basic FTP), verifying files post-transfer, and maintaining regular backups to mitigate the inevitable risk of data loss or corruption.