When you launch a block device driver (e.g., devb-mvSata), it detects the partitions on the block I/O devices. You can then issue mount commands to start the appropriate filesystems for various partitions.
You aren't likely ever to need to stop or restart a block filesystem; if you change any of the filesystem's options, you can use the -e or -u option to the mount command to update the filesystem.
If you need to change any of the options associated with the block I/O device, you can slay the appropriate devb-* driver (being careful not to pull the carpet from under your feet) and restart it, but you'll need to explicitly mount any of the filesystems on it.
To determine how much free space you have on a filesystem, use the df command. For more information, see the Utilities Reference.
Some filesystems have the concept of being marked as "dirty." This can be used to skip an intensive filesystem-check the next time it starts up. The QNX 4 and Ext2 filesystems have a flag bit; the DOS filesystem has some magic bits in the FAT. By default, when you mount a filesystem as read-write, that flag is set; when you cleanly unmount the filesystem, the flag is cleared. In between, the filesystem is dirty and may need to be checked (if it never gets cleanly unmounted). The Power-Safe filesystem has no such flag; it just rolls back to the last clean snapshot. You can use the blk marking=none option to turn off this marking; see the entry for io-blk.so in the Utilities Reference.