Disk Utility should be able to wipe out any existing partitions before attempting to install a new OS, but my favorite fallback is to use a Windoze PC to 'clean' drives to get back to basics before a new install. If you have access to (almost any PC with a USB port) and you have a USB to SATA cable to plug-in a bare drive externally, then I start by right-clicking the start menu to get to the legacy start menu and then select Disk Management. Disk Management is like Mac's Disk Utility, but I use it first to get an overview of the drive(s) before starting. The takeaway there is usually visible to the left where it shows you the disk number assignment(s) by the OS. The boot volume will usually be Disk 1 and the external (or other) drives will be 2, 3, etc.
While keeping Disk Management open, go to the taskbar and type: cmd to launch a terminal window (aka DOS command interpreter). At the prompt type: diskpart - which will usually open a new window with a different prompt. At that prompt type: select disk 2 (assuming your recycled drive shows as that number in Disk Management). Make certain that you have selected the correct drive (measure twice/cut once) and when confident type: clean and press return. At this point you can close the command prompt windows and the Disk Management window will now show the drive you just wiped as uninitialized. At that point you've blanked out the drive and back on the Mac you should see this reflected in Disk Utility. If you use Disk Utility to initialize the drive it would be advisable to start by formatting it as a GPT (GUID Partition Table) first (MBR is now the best option for bootable flash drives) and then creating Mac partitions as needed. You will probably want to use APFS for SSDs but Mac HFS works fine for spinning media.
My cMP 3,1 has a 1.92TB Samsung SSD that I have multiple partitions on. The primary (450GB) partition is now Sequoia, and I also have legacy Mac HFS partitions for High Sierra and Mavericks with the remaining space formatted as a Mac HFS data volume. This configuration is not without some problems. I upgraded the original GPU to an inexpensive used metal-compatible RX570 which OCLP provides the means of loading kext drivers so once the EFI partition is loaded at startup provides a boot picker that now allows all three OSes to display and load. I had originally used dosdude1s patcher to get High Sierra to load and ran it again on the Mavericks partition to get his kexts to let me almost use Mavericks. I say almost works because the RX570 is not actually recognised correctly by the OS and without the original GPU acceleration and VRAM some legacy apps (like Apple Compressor) do not work yet and I have little hope that there will be a solution other than swapping out the GPU with the original card if I need to go back.
While keeping Disk Management open, go to the taskbar and type: cmd to launch a terminal window (aka DOS command interpreter). At the prompt type: diskpart - which will usually open a new window with a different prompt. At that prompt type: select disk 2 (assuming your recycled drive shows as that number in Disk Management). Make certain that you have selected the correct drive (measure twice/cut once) and when confident type: clean and press return. At this point you can close the command prompt windows and the Disk Management window will now show the drive you just wiped as uninitialized. At that point you've blanked out the drive and back on the Mac you should see this reflected in Disk Utility. If you use Disk Utility to initialize the drive it would be advisable to start by formatting it as a GPT (GUID Partition Table) first (MBR is now the best option for bootable flash drives) and then creating Mac partitions as needed. You will probably want to use APFS for SSDs but Mac HFS works fine for spinning media.
My cMP 3,1 has a 1.92TB Samsung SSD that I have multiple partitions on. The primary (450GB) partition is now Sequoia, and I also have legacy Mac HFS partitions for High Sierra and Mavericks with the remaining space formatted as a Mac HFS data volume. This configuration is not without some problems. I upgraded the original GPU to an inexpensive used metal-compatible RX570 which OCLP provides the means of loading kext drivers so once the EFI partition is loaded at startup provides a boot picker that now allows all three OSes to display and load. I had originally used dosdude1s patcher to get High Sierra to load and ran it again on the Mavericks partition to get his kexts to let me almost use Mavericks. I say almost works because the RX570 is not actually recognised correctly by the OS and without the original GPU acceleration and VRAM some legacy apps (like Apple Compressor) do not work yet and I have little hope that there will be a solution other than swapping out the GPU with the original card if I need to go back.
