1. Read about EFS in Start, Help and Support.

2. A good compression or other tool never corrupts the data, but has been known to deny access to the data forever (see EFS!)

3. There are likely to be issues with files over 4GB, and even moreso on FAT32 volumes. No fix will be offered (ever!) for the FAT32 limitation.

4. There are (too) many ways to lock up files, but all carry the risk of data loss. EFS is a good example.