Fix ERROR_INSTALL_TEMP_UNWRITABLE 0x00000660

That "Temp folder is full or inaccessible" error is a pain, especially mid-install. Here's the fix I've used on dozens of machines.

Step 1: Clear the Temp Folder

Open Run (Win+R), type %temp%, hit Enter. This opens the user Temp folder. Select everything (Ctrl+A) and delete (Delete key). You'll see "file in use" warnings — skip those. The real culprit is usually a single huge file, like a log file or a failed download that's gigabytes.

Had a client last week whose entire print queue died because of this — their temp folder had a 23GB log file from a botched driver update. After deleting that, the install ran fine.

Step 2: Check Free Space on C:

If you can't delete the folder, or if you're still getting the error after cleanup, run Cleanmgr.exe (Disk Cleanup). Select C:, skip the usual junk (temp files, recycle bin), and go straight to System Files. There, check Windows Update Cleanup and Delivery Optimization Files. Those two alone freed 15GB on a client's laptop last month.

Why This Works

Windows Installer (msiexec) uses the Temp folder to extract and stage installation files. If it can't write a single byte, it bails with 0x00000660. It's not a permissions issue in 90% of cases — it's a full drive. The error text says "full or inaccessible," but in my experience, it's almost always full. The "inaccessible" part only happens if the Temp folder is somehow locked by malware or if you're running the install from a removable drive that got yanked.

Less Common Variations

Variant 1: System Temp Folder (not user Temp)

Sometimes Windows uses C:\Windows\Temp instead of %temp%. Open that, delete contents (same warnings about files in use). This happens with old installers designed for Windows 7 era — they don't respect user Temp paths. If you've already cleaned user Temp and still hit the error, check this folder.

Variant 2: The drive isn't C:

Rare but possible — if you set the TEMP and TMP environment variables to a different drive, and that drive is full. Check with echo %TEMP% in Command Prompt. If it points to D: or an external, clear that drive or change the variables back to C:\Temp.

Variant 3: Corrupted User Profile

This one's a long shot but I've seen it twice. If the user profile is corrupted (signs: apps crash, settings don't save), the Temp folder path might be wrong. Quick test: create a new local user account, log in, try the install. If it works, migrate your data and ditch the old profile.

Prevention

Set a recurring cleanup schedule. Use Task Scheduler to run cleanmgr.exe /sageset:1 once a week. That's the command to save cleanup settings. Then schedule cleanmgr.exe /sagerun:1 weekly. Takes 2 minutes to set up, saves you from this error forever.

Also, never leave game or app installers sitting in Downloads for months. They expand into Temp and never clean up. I've seen Steam installers leave 10GB+ of garbage. Delete downloads after installation.

If you're on a laptop with a small SSD (128GB or 256GB), move your pagefile.sys to a secondary drive — that alone can free 8-16GB. Right-click This PC > Properties > Advanced system settings > Performance > Advanced > Virtual memory. Set C: to "No paging file" and put it on D: or another drive. Restart. That's an intermediate tweak but it works.

Bottom line: 0x00000660 is almost always a space problem. Don't mess with permissions or registry unless you've cleared space and it still fails. 9 times out of 10, deleting Temp folder contents fixes it in under a minute.