Hey guys, Recently I reformatted my hard drive and put a clean install of Windows 7 onto it, but I'm having a problem with a folder on my external drive now (note: I didn't reformat the external drive and the OS I had before was also Windows 7). On my external drive I have a folder labeled, "Media". Somehow the folder's security settings have been messed up. There are now two unknown accounts under the Security tab that I cannot delete (nor can I even add another user). The general names of these unknown accounts are "S-1-5-21-*". I also found the accounts on a few other folders on the drive, but I was able to delete those. The Media folder is the last folder (as far as I'm aware) that has these accounts under the Security tab. This is quite a problem because due to these accounts I cannot write into the folder. I can read everything, but I cannot write. I'd really rather not reformat the drive because I have roughly 500GB of data on the drive and I do not have anything else that can hold that much information. I suppose one option could be to create a new folder and transfer the data into it, but I wouldn't be able to delete the original Media folder afterward so I'd rather find a way to eliminate the accounts. Thanks in advance for any help! EDIT: After some further investigation I discovered that the subfolders of the Media folder also have these unknown accounts under their Security tab. I also went into the registry editor and attempted to delete the user accounts, but I keep getting an error that says they cannot be deleted. If I dive deeper into the registry editor I can delete certain folders under the account's registry entry, but if I attempt to delete one that cannot be deleted (I receive an error for), all of the other folders I deleted will return. This sounds like some sort of malware. Should I be concerned?