That is normal because the device is trying to re-entrying all of the files (mp3, pictures, videos) in to the database. It re-syncing all the list every time you mounting it as Mass Storage. The problem is in the Media Player on the device and the Photo Viewer.

The more files you've got on the eMMC or Memory card is the longer it take to relisting them again. This is a flaw of the N900 within the OS itself.

To solve the problem, try reboot the device a few times until the CPU load is no longer stay maximum.

I hope this issue will be fixed in the PR1.2