I'm looking for a reliable backup solution on Windows, something with a feature set similar to Yadis.
I've been using CrashPlan for 2 months, but their software lost more than 1TB of my data, that's why I'm looking for alternatives.

Requirements:

• Real-time folder-to-folder backup:
  I don't need online features, I want to use this to duplicate my files between my local disks.
• Versioning support:
  Should be able to choose how many versions to keep of the files.
• Plain backup:
  I'd like to be able to open the backup without special software. No proprietary file format.
• External disk support:
  Shouldn't have any problem using external disks, at least on the source side.
• Backup every file, even locked/system files. (Yadis fails this one)
• Should start automatically, and have a comfortable GUI.
• Should use actively maintained and/or popular software. I don't want to use discontinued products.

Optional requirements:

• Low RAM usage
  I'm not really comfortable with something that eats 1GB of my RAM.
• Compression support
  Preferably ZIP, but I'm not picky about this. Any ordinary everyday format is acceptable.
• Freeware or Open Source
  Preferred, but not necessary. I can do a one-time payment within reasonable bounds. (preferably under $100)
• I only need Windows support, but if it works on Linux, that's a plus.

I've already searched and tried lots of software, most of them failed at the plain backup, the versioning or the locked file backup requirements.