MacOS – Files disappeared after Mavericks upgrade

There is no need to log in to individual accounts, at least with more modern versions of OS X. You can set access privileges for managed accounts from within the Users & Groups pane of System Preferences without leaving your own account.

By selecting the Other tab in the Parental Controls dialog you can even enable Remotely managing parental controls.

Upon further clarification by Cra, it seems that Apple's file quarantine system is causing the difficulty. Starting with Mac OS X 10.5, files downloaded from the internet are quarantined and checked for safety.

Applications that are downloaded must be approved the first time that they are run, and only the owner of the account used to initially download the software can grant that approval.

If you have multiple user accounts on your Mac, the user account that downloaded the file is the only user account that can remove the quarantine attribute to the file. All other user accounts can open the quarantine file, but they will be presented with the quarantine dialog box asking "Are you sure you want to open it?" every time they open the file.

ᔥ Apple Knowledgebase:About file quarantine in Mac OS X v10.5 and v10.6

Make sure that you download software updates using your administrator account and that you open and approve the software from your account before making any adjustments (which may not be necessary at this point) to the Parental Control settings. It may be necessary to disable automatic updates in Firefox.

The quarantine system works by attaching an extended attribute to a downloaded file. If absolutely necessary, you can remove this attribute using the terminal

Enter the following command, replacing appropriately:

xattr -d com.apple.quarantine /applicationPath/<applicationName>.app

See also: Stack Overflow: How do I remove the “extended attributes” on a file in mac

Since the problem followed the hard drive when connected to a new computer, you have a clear case where the problem is disk based. The slowness, the error, and that it seems to be getting worse would make me think that you may only have a short amount of time to finish your recovery efforts.

Worst case, the drive itself is failing and you can only get files off while it cooperates. Best case, the file system is corrupt and once you zero the drive entirely, it will work properly. The test that determines which of the two is the case involves erasing the entire drive, so plan for the worst and hope for the best.

1. Consider stopping your recovery attempts if the data is of value such that you would pay a professional. If it is hardware, time may run out and running the drive could make things worse.
2. Prioritize and get the most important files off that drive entirely considering both the Mac side and the Windows side.
3. Use PC file recovery tools on the Boot Camp side, use Mac recovery tools like Data Rescue 3 on the Mac side. Also, don't overlook tools like rsync that can ignore errors and keep trying to copy other files after running into the initial error - 36.

Good luck. It seems like you have a good handle on various options for getting files off the drive so I wanted to offer some other options. Also, you made no mention of backups whatsoever - depending on how recent your last windows backup and last Mac backup are, you could reduce the scope of your efforts accordingly.