If you’re getting an error while installing the Windows 10 Technical Preview (for me it was under build 9926) that says something like “we couldn’t update the system reserved partition” most likely your system reserved partition doesn’t have enough free space on it. I ran across a number of articles regarding Windows 8 preview editions and found this Windows 8.1 post about a similar issue.
I tried the other methods in that guide which involved the following…
1. Press Win + R, type diskmgmt.msc and click OK.
Right click the boot partition listed at 100MB in a map, select Change drive letters and paths > Add, and choose Y: for it.
2. Press Win + X, select Command Prompt (admin), and run the commands one by one:
takeown /f . /r /d y
icacls . /grant administrators:F /t
attrib -h -s -r bootmgr
3.Open File explorer, go to Y:\Boot, delete all except en-US. languages.
4.Run command again:
chkdsk Y: /F /X /sdcleanup /L:5000
5. Now you can start the 8.1 installation. After upgrade has been done, go back to remove the drive letter.
In the end though what worked was Mini Partition’s tool which allowed me to resize the partition. There was a little nail biting but Windows rebooted and now I appear to be on my way to a healthy update.
edit: You should also have luck using something like YUMI and a Linux distro + USB drive.