corruption on external disk

The problem with advanced SSD drives, when used with iPad, is that of the drive design.

For both high performance and minimising of write-cycles (that shorten the life of the drive), some drives employ internal write-buffers - the purpose of which is to allow data to be efficiently written to the drive in blocks. Whilst the host computer believes that it has stored data to the drive, the internal drive controller often retains data in its buffer until a full block can be written to the drive. Until such time as the controller purges this buffer to flash storage, any data in the buffer will be lost if unexpectedly disconnected from the host computer.

For this reason, unless the External drive is configured for “quick removal” (effectively disabling internal buffering), the drive needs to be warned (by the host) of an impending dismount - allowing adequate time for buffered data to be written to non-volatile storage. On Windows PC, this warning is facilitated by a manual dismount from the filesystem prior to disconnection of the drive. By contrast, if the drive is configured for quick removal, the drive can be disconnected at any time that the drive does not indicate activity.

A drive that is manually dismounted will typically set a “clean switch” - that, upon reconnection, indicates to the host that the drive was correctly dismounted without corruption. If the “clean switch” is not detected upon mount, the OS assumes that the drive must be checked and corruptions repaired prior to a successful “mount” operation.

iPadOS doesn’t provide a “dismount” mechanism or an ability to mark the drive as clean - nor can it scan and correct drive errors. As such, an attached filesystem is assumed to be capable of “quick removal”.

It follows, therefore, that if an external drive is not configured for quick removal, then write-errors/corruption is likely to occur.