Drive letter disappear after reboot

I think there are still two keys in the registry that control mounted storage devices. I think what may have happened is that the key that records the disk "signature" and the key that relates the drive letter to the appropriate "signature" got discombobulated. So he could assign a letter when it was running, and it would stick, but not written to the registry because of the disconnect between the two keys. So after a reboot he'd have lost his drive letter all over again.

So when the the disk's MBR/PBR is overwritten with zeroes, and then Windows sees it, it thinks "new disk" and rewrites the correct data to those two keys in the registry. Now the info in each registry key properly corresponds with each other and he can assign a letter which will stick across reboots. And the more I think about it, the more I think that all that was necessary to fix it would be to delete the one or both of the keys that correspond with that disk. But, without seeing the registry before/after it would be hard for me to say for sure.

I don't think there was any corruption of the MBR/PBR that caused this, or anything wrong at all with the drive, as a drive has no idea what drive letter has been assigned to its partitions. If there was something wrong with the MBR/PBR, he would have had problems beyond the assigning of drive letters.

And yes, agree with you that he would've lost all data on the disk. Unless he used recovery software to get the data within the first and last million sectors. But I think you had given plenty of warning.

sibbil said:

I think there are still two keys in the registry that control mounted storage devices. I think what may have happened is that the key that records the disk "signature" and the key that relates the drive letter to the appropriate "signature" got discombobulated.

Now he did go through the DISKPART steps I described for him, to show the disk signatures for the two drives. And they were different, which of course would have been expected since both drives were also shown as ONLINE. Had their signatures somehow gotten to be identical that would have been a "collision" and one of the two drives would be shown as OFFLINE.

But I suppose your theory seems plausible (if inexplicable) that the Registry keys for the mounted devices somehow got screwed up for the E drive. This really is the unanswered explanation we're looking for.

So he could assign a letter when it was running, and it would stick, but not written to the registry because of the disconnect between the two keys. So after a reboot he'd have lost his drive letter all over again.

I honestly don't know enough about this to understand how the drive letter can be assigned while Windows is running, but not also update the Registry key.

But if that's what is going on I guess it's the explanation for why the assigned drive letter didn't last across boot.

So when the the disk's MBR/PBR is overwritten with zeroes, and then Windows sees it, it thinks "new disk" and rewrites the correct data to those two keys in the registry. Now the info in each registry key properly corresponds with each other and he can assign a letter which will stick across reboots.

There still are some unanswered "loose ends" in my mind to this hypothesis, but it does seem to be the explanation. It certainly worked for the current situation, and it also worked for that original poster on the other thread I found. Wonder who told him to do that "zero fill", or at least try it, or if it was just blind luck out of some general discussion and suggested ideas of things to try... and that one actually DID work?

And the more I think about it, the more I think that all that was necessary to fix it would be to delete the one or both of the keys that correspond with that disk. But, without seeing the registry before/after it would be hard for me to say for sure.

I agree with this supposition.

I don't think there was any corruption of the MBR/PBR that caused this, or anything wrong at all with the drive, as a drive has no idea what drive letter has been assigned to its partitions. If there was something wrong with the MBR/PBR, he would have had problems beyond the assigning of drive letters.

Agreed. But then we really don't have a bullet-proof explanation for what type of Registry corruption occurred regarding disk signature.

In other words, in one place or another "something was wrong" (how's that for real science!), which we don't know how it got to be that way. But by essentially forcing DISKMGMT to see the drive as "brand new and initialized" it caused Windows to simply replace whatever corrupted or invalid Registry data was previously present, with brand new and valid and consistent values... and CASE SOLVED!

And yes, agree with you that he would've lost all data on the disk. Unless he used recovery software to get the data within the first and last million sectors. But I think you had given plenty of warning.

Yes. that original thread never mentioned the fact that the low-level format of the drive (either the first million sectors, or the whole drive) would obviously have initialized it to empty, losing everything previously on it. Yes, at least a drive letter could now be assigned to the newly created partition(s) and it would stick across boot, which at least gets past that major issue. But the cost was a total loss of data on that drive, which could either have been of no consequence or he had a backup to restore from, or who knows. But it's obvious all data had to be lost by that "zero fill" action.

And the same with the current story. Yes, the drive letter problem is finally fixed. But once again, the cost is the erasure of the drive... which may or may not be of any consequence. If there was meaningful data there, it was either restored from a backup or lost forever.


Honestly... this has been remarkably enlightening. Up until this discussion I'd only come across the "disk signature collision" situation as why a drive would be unlettered and could not be given a letter... but indicated as OFFLINE. I've never heard of this current symptom, of a "temporarily assignable drive letter that does not persist across re-boot".

Fascinating discussion. And informative.

Again, real credit goes to that original poster who provided the actual solution... and to "The Google" for allowing me to find it.

I'm actually having the exact same problem with Windows 7 Pro with (2) SATA drives that are running as a RAID1.

I'm stumped and can't afford to write zeros to the drives to try and fix it. I suppose I need to buy a new back up drive and back up the RAID and start fresh.

I did have this RAID die once before and I'm starting to wonder if that is the culprit.

nick james said:

I'm actually having the exact same problem with Windows 7 Pro with (2) SATA drives that are running as a RAID1.

I'm stumped and can't afford to write zeros to the drives to try and fix it. I suppose I need to buy a new back up drive and back up the RAID and start fresh.

I did have this RAID die once before and I'm starting to wonder if that is the culprit.

i sorry to hear that. but my drive was fixed after write zero. i don't know other way that would fix this stupid problem. i still don't know what is the main cause but one might be improper formatting of the drive. DO NOT USE the format that was included in Win 7 system drive or very old formatting disk( thats what i used). i can write zero because my system is new and the win 7 was installed on my ssd, my hard drive was empty in the beginning. well i hope you find a way other than write zero. if you do please post back.

owen0729 said:

nick james said:

I'm actually having the exact same problem with Windows 7 Pro with (2) SATA drives that are running as a RAID1.

I'm stumped and can't afford to write zeros to the drives to try and fix it. I suppose I need to buy a new back up drive and back up the RAID and start fresh.

I did have this RAID die once before and I'm starting to wonder if that is the culprit.

i sorry to hear that. but my drive was fixed after write zero. i don't know other way that would fix this stupid problem. i still don't know what is the main cause but one might be improper formatting of the drive. DO NOT USE the format that was included in Win 7 system drive or very old formatting disk( thats what i used). i can write zero because my system is new and the win 7 was installed on my ssd, my hard drive was empty in the beginning. well i hope you find a way other than write zero. if you do please post back.

I will... For now it sounds like I might just be living with it until a hard drive upgrade!

sibbil said:

Hey Nick! Welcome!

It's been a long time since I messes with RAID, so I would be hesitant to make any recommendations that deal directly with the drives.

However, there may be something in the registry that might be done. Do you feel comfortable going in there to take a peek at something? Do you know how to back up and restore it?

I think so?
start > cmd > regedit
then export it
and to restore it, import right?

Thanks sibbil!

Yes, and better yet is to create a restore point. See:

Registry - Backup and Restore

Here's something I had previously written based on what I saw in this thread:

Regarding the thread where the user was losing his drive letter across boots: this is the regkey I was speaking of: HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices.

If you look at that key, you see a "signature" subkey for every volume that's been mounted. If you look at the data for that subkey you'll see a hex value. That hex value corresponds to the data of another key in the same branch. The name of that subkey with matching data contains the drive letter for that partition.

I'm guessing what may have happened is the hex value that's common between the two keys got mismatched somehow. So when rebooted, Windows looked at the hex data for the volume signature, and found no matching hex data that matched with a drive letter.

Wiping the first sectors, killing the MBR/PBR data, repartitioning, would force Windows to recreate the needed subkeys.

In your case with RAID, and not knowing how it's being managed, I would suggest only examining the contents of that key and reporting back with what you found.

I'm guessing you'll find the subkey that matches the volume in question with an assigned hex value. It should be there if the OS really sees the volume.

Then look for the subkey that's supposed to match the hex value with a drive letter. I'm guessing you may find a key for the intended drive letter but the hex data for it doesn't match the other key.

Please see my attached screen shots lol. I took two because all the values were not viewable on one screen

I have a feeling something doesnt look right here.

Looks as you've had quite a few devices attached in the past. Unsure what those keys starting with the "#" symbol are. Haven't seen those before.

It's a two disk RAID1?

You've got to widen the data column to show all in order to match the hex values. It's hard to do if you don't know info on the drives/volumes connected. It might be best to export the MountedDevices key to a text file so that it can be imported into Excel for sorting.