1 TB SSD not showing up in "start up disk".

[Kipsley]

Hey everyone. I have an old Mac Pro (mid 2012) running Mojave 10.14.6
My problem is with my 2 SSD drives (both are bootable) one is 1TB, the other is 250GB.
The 250GB SSD shows up in "start up disk" where I can choose the disk to boot from, but the 1TB SSD does not show up.

The difference is that the 250GB SSD is GPT (GUID Partition Table), while the 1TB SSD is MBR (Master Boot Record), and I have discovered that that does not let it show up in "start up disk" even though not works just fine.

So then how do I get my 1TB SSD to be GPT (GUID Partition Table)?

 

[Bigwaff]

So then how do I get my 1TB SSD to be GPT (GUID Partition Table)?

The easiest way is to erase and reformat the SSD using Disk Utility. Make sure to select GPT.

 

[Kipsley]

Disk Utility doesn't give me an option for selecting GPT.
I get APFS, and Mac OS Extended, MS-DOS (FAT), and ex-FAT, but these options are only available when asking to erase a HDD.
The SSD options are only Mac OS Extended, and MS-DOS (FAT), and ex-Fat. No APFS options.

 

[MBAir2010]

Disk Utility doesn't give me an option for selecting GPT.
I get APFS, and Mac OS Extended, MS-DOS (FAT), and ex-FAT, but these options are only available when asking to erase a HDD.
The SSD options are only Mac OS Extended, and MS-DOS (FAT), and ex-Fat. No APFS options.

after erasing the ssd, one can select APFS on the blank drive in Mojave when formatting.

 

[Bigwaff]

Disk Utility doesn't give me an option for selecting GPT.
I get APFS, and Mac OS Extended, MS-DOS (FAT), and ex-FAT, but these options are only available when asking to erase a HDD.
The SSD options are only Mac OS Extended, and MS-DOS (FAT), and ex-Fat. No APFS options.

Those are file system format types. Make sure you select the following menu in Disk Utility.. View > Show all devices ... afterwards, select the top most level "device" representing your external drive and "Erase". You should be prompted to pick a partition map type.

Erase a volume using Disk Utility on Mac

In Disk Utility on your Mac, erase a volume so you can use it again.

support.apple.com