How to flush cache of hard-disk and flash-disk (or filesystem) from command line?

I'm writing a backup script which

1. Copies the data to backup disk.
2. Flushes the backup disk.
3. Performs a hash integrity check.

Before I used to do sleep(60) for waiting a minute so that data is automatically flushed by the kernel. Which I guess is overkill so now I'm trying sudo hdparm -F --verbose /dev/disk but it reports error - HDIO开发者_开发技巧_DRIVE_CMD(flushcache) failed: Invalid exchange multiple times.

I'm wondering is there any standard way to flush the cache to hard disk. I think there is because usb-creator-gtk does it, umount does it.

I'm using Ubuntu x64 9.10

PS: I'm trying to avoid "sync" because this page says that it is not safe. http://ubuntuforums.org/showthread.php?t=589975

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Does sync suffice?

Edit: regarding your edit - you are trying to avoid sync because some guy on the internet put a CYA disclaimer on his post? Maybe there is something wrong with sync of which I am unaware but that might be worth a 2nd post in itself.

Still, from the linux info pages:

sync writes any data buffered in memory out to disk. This can include (but is not limited to) modified superblocks, modified inodes, and delayed reads and writes. This must be implemented by the kernel; The sync program does nothing but exercise the 'sync' system call.

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You want fsync (man section 2) function call, but if you're doing this in a script, you'll probably want to use the sync command (man section 8), which just calls sync().