I’ve reported this to support, but it’s probably worth other users knowing, because of the security and space implications. Investigation of the iXpand directory on a jailbroken iPad shows that the tmp directory retains a copy – in fact appears to retain two copies – of every file copied between the device and the iXpand. These copies persist across version updates, and the manual Clear Cache function doesn’t delete them. This appears to be the case for all iXpand models and all iOS devices and system versions, and today’s update to the iXpand app (5.0.4) hasn’t fixed it.
Obviously this has fairly significant consequences for security (copies of all your deleted files remain in onboard memory) and storage (since your device memory is being filled up with multiple copies of the precisely the files you’re offloading to the iXpand in an effort to save space). The persistence of cached copies between app updates also turns out to have been the cause of a longstanding problem with battery drain and device overheating I reported in another thread. You can still get rid of the cached copies by deleting and reinstalling the app, but it’s a bit of a chore to have to do this every time you move something confidential off your device; with luck it’ll be sorted in an update, but it seems worth alerting users to the issue.