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how much does Bit tornado download before it is saved to the hard drive because i can't keep it running constantly and when i open it after it is closed, the amount i have downloaded has reduced.

It varies by filesystem and by torrent.

The torrent's piece size determines the save interval.
The reason your downloaded amount has reduced is that some of the downloaded amount from the previous session was partially completed pieces.

Piece size varies by torrent.

In cases of crashes in the client or operating system, FAT32 will have more loss than other filesystems like NTFS (on windows).