Note - Partitioning a drive will also reformat the drive.

Disk Utility divides the space in half by default.

The slider button is just above the +/- buttons.

Using the slider on the pie graphic, change the size until your partition is as big as you want it.

Sometimes, using other formats will mess up your hard drive. It is very strongly recommended to use APFS.

Give your new partition a name, and then select the format.In the pop-up, the utility asks if you want to create a whole partition or just an extra volume.Next, click the + (plus) button underneath the pie graphic to create a new partition.On the next screen, click the Format box and switch it to APFS.At the top of the screen, click the Partition button.Select your external hard drive on the left margin.Double-click Disk Utility when it pops up. Open Finder and type Disk Utility into the search bar.You can use other formats, but it gets a little wonky. Connect the drive you want to partition.Worst case you'll rule out the SATA connection, but this worked for me.Also, if you do intend on using your external hard drive to boot Windows, Apple strongly recommends using Boot Camp to partition the drive instead of the Disk Utility feature. If you're having this problem trying formatting the new HD via a USB/SATA cable (instead of in your macs HD bay) before you return the drive or give up. I'm going to replace the SATA cable just to be sure, but I think this may be the problem for others with older MBPs who are trying install a new drive. It's been a day since I installed OSX on the new SSD and I haven't had any problems. I think that would explain why unplugging the original HD and plugging it back in allowed it to boot once. After playing with the SATA cable a bit just to make sure it had good connections I was able to format the drive and install 10.6 with no problems. I bought and installed a new SSD but wasn't able to format/erase due to the same 'cannot allocate memory' error using Disk Utility or Terminal. Oddly enough when I disconnected the HD and plugged it back in, it fully booted before crashing, then back to the original failure to boot every time after. Orignal HD quit and I couldn't boot under any mode. I know this post is old but I was having the same problem with my 2009 MBP and thought others may be too.

Mac os format hard drive