Media Uploader – partial file uploads occuring only with Mac OS X

We are encountering a persistent problem when using the WP media uploader (either the multi-file uploader, or the browser uploader). Approx. 50% of attempts, the file does not upload completely. This behavior is limited to staff using Mac OS X Version 10.7.3. This behavior has been replicated on three similar Macs in our office.

To check for browser issues, the problem has been replicated using current versions of Firefox, Safari, as well as Chrome.

This behavior has NOT been replicated using a Win7 machine, from which the upload process works flawlessly.

We have set up a test WP site (3.3.1) with no plugins installed and using the default theme, and the problem is the same.

The test site appears to rule out any issues with plugins or the theme.

The test site also appears to rule out any issues with the hosting provider, since it was hosted on a virtual host (hostgator), whereas the production site is hosted on a dedicated box (rackspace).

I have posted a screenshot that may be of interest, at https://cssselect.net/misc/pdf_upload_problem.png – on the right side of the ftp client, we see a list of pdf files on the server after multiple attempts to upload via the WP media uploader. The light green shaded area is 10 consecutive uploads of the same file from a Win7 machine, and the other files represent attempts from a Mac. As can be seen by filesizes, about 1/2 of the Mac attempts are incomplete uploads.

Any thoughts or info would be appreciated – it does not make much sense that this issue is related to the local OS, but from this testing, it appears to be the case.

Other than this issue, there are no other problems working with the WP backend from these Macs.

Regards,
Dave Seldon