Is it possible?
Is it possible?
In theory yes, as this what CHKDSK does. There are also a number of third party repair/recovery tools on the market that run under Windows, and claim to do this.
My personal view (and it is a generalisation) is that repair tools are more likely to be effective if you run them in safe mode.
Obviously a lot depends on what is actually wrong with your NTFS.
In general, you want whatever tool you use to check for any NTFS errors to have exclusive access to the system. With Windows (after the start-up sequence finished), this becomes a bit troublesome because all kinds of applications will be requesting the services from this device. This tends to slow down the repair system because all these other tasks are intervening with it. Plus, the repair tool has to make sure that all other application still get their proper data. And more troublesome, some tasks might have access to a file, and because it is open, the repair tool might not have full access to it until it is closed again.
Doing repairs during the start-up sequence has one major advantage, though. Almost everything on the harddisk will still be closed and the repair tool can claim exclusive access to the disk. Even better, all other tasks will just wait for the repair tool to finish what it is doing. Thus it can repair faster and more easily. It just needs to reboot.
Technically, it would be possible to use Windows in some exclusive mode after it is running but I think Microsoft kept that out of the OS on purpose. A badly written application could for example enter this exclusive mode and then end up in an endless loop and the system would just be dead until someone switches the power off and on again.
Fortunately, many repairs can be done already while Windows is running and don't need a reboot. Partition Magic, for example, can divide any harddisk in multiple partitions for as long as you haven't accessed it yet. (Meaning, the drive has no drive-letters assigned.) Basically, like Linux, Windows is mounting and unmounting disk stations. But with Windows, a mounted disk gets a disk letter. But once mounted, Partition magic has to temporarily unmount the disk so no other applications can get access to it. If this happens to be your disk that your OS is running on, this would be a problem. Thus PM partitions mounted disks only during the start-up sequence, just like many repair tools do. That way, PM has exclusive access and can thus move around clusters and split (or merge) the disk partitions.
So the answer:
Theoretically it is possible. It just takes a lot of work and it has some risks of messing things up. Doing these actions during reboot just makes things easier and a bit faster, although it does require a reboot.
Thanks everyone. That answers my question completely.