I boot via a portable SSD and use that as my main drive. I've been unable to install Mojave as it says "GUID Partition Table (GPT) required?
iMac (21.5-inch, Late 2013), macOS High Sierra (10.13.6)
Explanation:
The modern mac OS's create a number of partitions on the startup disk: RecoveryPartition, EFI partition and at least one OS partition. GUID Partition Table is the only possibility to create 4 or more partitions (the alternative Master Boot Record, can handle max 4 partitions but goes wrong when they are differently formatted, HFS+, APFS, Dos ….). That's why macOS asks for GUID Partitition Table.
I suggest you to keep your Time Machine backup updated, make a First Aid pass to the Time Machine (just in case) and go for it. I don't know how much memory you use on your SSD, but if you have enough free space on your Mac can be a good option fo course.
Ok, thanks for your reply. I use Time Machine, however before I format the disk/volume, should I make a copy from the portable SSD to the original Mac hard disk as a precaution?
Thanks again.
You may need to repartition the disk because it may not have the partition scheme required. You can install macOS only on disks with the "GUID Partition Table scheme".
You can use Disk Utility to check the external disk’s partition scheme, and format the disk/volume destination.
Cannot install Mojave as GUID Partition Table (GPT) required.
