How to Fix: Cannot Create Partition: No Free MBR Slots

Infopackets Reader Sam T. writes:

" Dear Dennis,

I have an HP computer, which has 4 hard drive partitions: the System partition, the C drive, the factory image partition (which includes a backup of Windows), and the 'HP Tools' partition, which contains BIOS information in case a BIOS flash fails and I need to restore my BIOS. My question is this: I used MiniTool Partition Wizard to shrink my C drive, which then created unallocated space. Normally I can right click the unallocated space and create a new volume, however, I receive an error message that says: 'Partition Wizard cannot create a partition here because there are no free MBR slots'. I don't know what that means or what to do next. Can you help? "

My response:

I use MiniTool Partition Wizard from time to time - it's good, and it's free - but, I have to say that I have never heard of this issue before. I asked Sam if he would like me to connect to his system and look at the issue in depth, and he agreed. Here's what I found:

First, I launched Disk Management within Windows and right clicked over top of the unallocated space Sam had created earlier, but received an error stating that: "You cannot create a new volume in this unallocated space because the disk already contains the maximum number of partitions." I then tried to do the same using MiniTool Partition Wizard and was also greeted with the same error Sam received earlier - there were no free MBR slots.

I did a bit of research, and what it boils down to is this: Sam had 4 partitions, and all of them were set as primary - which happens to be the maximum number of primary partitions you are allowed per disk. This information is then stored in the MBR (master boot record) of the hard drive. With that said, most people won't run into this issue because most standard Windows installations will only contain 1 primary partition, which is located on the C drive. However, Sam's computer was from HP and they do things a little differently.

How to Fix: Cannot Create Partition: No Free MBR Slots

After a bit more research I discovered that it is technically possible to convert one of the primary partitions into a logical partition, thus abiding by the laws of the master boot record (MBR). However, I advise not going this route, because it will likely break the functionality of the computer down the road. In Sam's case, the Recovery and HP Tools were both primary; changing either one to logical may break either the factory image restoration or the BIOS recovery - and you would not want to find out that either one of these recoveries doesn't work when you need it the most.

A better way to go about this problem is to work with what is already there. In Sam's case, the hard drive layout looked like this:

System (Primary) | C Drive (Primary) | Unallocated Space | Factory Image (Primary, Backup) | HP Tools (Primary)

What I did was use MiniTool Paritition Wizard to merge the Unallocated space (which was originally taken from the C drive) with the Factory Image immediately next to it, and then marked all the files on the Factory Image drive as hidden. That way, Sam could use the newly repurposed partition for whatever he wanted, and technically this did not break his pre-existing partition schema. Sam's hard drive layout then looked like this:

System (Primary) | C Drive (Primary) | Sam's new partition (Primary, Still contains Factory Image, but now larger than previous) | HP Tools (Primary)

I hope that helps anyone should they encounter a similar issue, especially with HP computers.

A word of warning: always make a disk image backup of your entire hard drive before you go changing around your partitions. I have used MiniTool Partition Wizard previously and adjusted the C drive geometry, only to have it crash and corrupt Windows, thus making the entire system unbootable. As always; if you don't know what you're doing, you are welcome to contact me for help - see below.

Additional 1-on-1 Support: From Dennis

If all of this is over your head and you need help partitioning your hard drive, I can help using my remote desktop support service. Simply contact me briefly detailing your problem and I'll do my best to get back to you as soon as I can.

Got a Computer Question or Problem? Ask Dennis!

I need more computer questions. If you have a computer question - or even a computer problem that needs fixing - please email me with your question so that I can write more articles like this one. I can't promise I'll respond to all the messages I receive (depending on the volume), but I'll do my best.

About the author: Dennis Faas is the owner and operator of Infopackets.com. With over 30 years of computing experience, Dennis' areas of expertise are a broad range and include PC hardware, Microsoft Windows, Linux, network administration, and virtualization. Dennis holds a Bachelors degree in Computer Science (1999) and has authored 6 books on the topics of MS Windows and PC Security. If you like the advice you received on this page, please up-vote / Like this page and share it with friends. For technical support inquiries, Dennis can be reached via Live chat online this site using the Zopim Chat service (currently located at the bottom left of the screen); optionally, you can contact Dennis through the website contact form.