Have you tried using System Restore to a restore point BEFORE you had a problem ?
Digital Rights Management was never intended to be the users friend.
Perhaps DRM has been offended and is teasing you with only 10% of your original 30 GByte.
I know a man afflicted with an IPOD using daughter. She had 60 Gbytes until a minor iTunes upgrade.
When we last spoke they were still waiting to see if they could re-download for free;
I doubt they will even get a discount coupon ! !
It is possible that if IPOD or iTunes are running, they will have locked you out of access to their registry keys.
The solution is to close down every wretched Apple thing and try again.
Originally Apple iTunes installed "iPod Service", plus "Bonjour service", plus one or two other services,
and all these services were configured as Startup type = Automatic.
i.e. APPLE starts and takes control of Windows as soon as I log-in. I really did not want to know that.
Perhaps you have the same affliction.
Even "nice" software leaves traces behind when asked to un-install, that is why Revo Un-installer exists.
Viruses don't un-install, they just find some-where else to hide.
If you do not know into which category I place Apple - you have not been reading ! !
So I re-imaged the disc back 2 hours to the happy state where it had never seen Apple.
Because I too have an IPOD using daughter, and also a son who works in I.T., I awoke from an afternoon siesta to find iTunes was once more installed together with the iPod Sevice ! !
But at least all the other maggot ridden Apple stuff had been discarded, and iPod Service startup type = Manual;
so it never ran in my profile, and it still "started" when my daughter was logged in and plugged her IPOD into the USB port (with the External Drive safely removed).
You should look in Services for ALL Apple Services (including "iPod Service" and Bonjour Service and any other rubbish).
If any Apple service is Started, that could have blocked their registry keys.
The solution is to select each of them in turn, then Right Click => Properties => General => Startup type = Disabled;
then reboot the computer and Windows will no longer be crippled by Apple interference;
then try again to restore the registry.
Once fixed you can always use Services to "Start" a disabled item;
or change its Startup Type to Manual in which case it only starts when you "need it".
If you still have a problem, wait to see any other suggestions.
If nothing works and you decide to reformat, then you will not have much to lose by meddling with the registry.
Anything which follows could lose all your music. Backup before you proceed.
What keys / data EXACTLY "cannot be written" ?
If your registry clean-up resulted in several *.reg files, then try to restore each in turn to decide which have a problem.
Having got a *.reg file with a problem, inspect its contents with Notepad.
If it has more than one key, use Notepad to create a new variant for each key.
e.g. the following has two keys, each with 3 names and values, and would have to be split into two new variants.
NOTES
Each variant must start with REGEDIT4, which MUST be followed by a blank line
The final name and value MUST be followed by a blank line.
REGEDIT4[HKEY_LOCAL_MACHINE\SYSTEM\Software\Comodo\Personal Firewall\AppCtrl\IPC\8\0]"Filename"="W:\\Program Files\\Mozilla Thunderbird\\thunderbird.exe""CRC32"=dword:aa8830c6"AccessRight"=dword:00000200[HKEY_LOCAL_MACHINE\SYSTEM\Software\Comodo\Personal Firewall\AppCtrl\IPC\8\1]"Filename"="C:\\Program Files\\CCleaner\\CCleaner.exe""CRC32"=dword:8cf95e9a"AccessRight"=dword:00000200
Then you can try to restore each variant in turn to determine which has the problem.
Then you can post the text here for further advice, alternatively :-
Registry Rules :-
1. Keep Out
2. Look but don't touch
3. You get what you deserved.
Sometimes I break all the rules, BUT I create a fresh ERUNT registry backup BEFORE I launch RegEdit etc.
If you do what I would do, you may be lucky.
If you get stuck it will be difficult to get you out of the mire with forum remote control,
but hopefully you should know your way around the registry if you are happy to re-install Windows.
I am just concerned that I should not lead any-one into a minefield.
When a registry key is not accessible, it can be made accessible by "taking ownership" of the key.
RegEdit can do this, but it is a real pain with many hurdles to overcome.
I prefer Registrar Registry Manager 5.51 - far less stumbling in the dark.
You can get the latest version, 6.00, as a free download (rrtri.exe) from
http://www.resplendence.com/downloads.
Once you have taken ownership, you should be able to restore the registry backups.
Regards
Alan