Choice of Scheme when formatting USB flash drive in macOS When using Disk Utility 16.3 with macOS Sierra to format a "thumbdrive" USB 3 Lexar flash drive (64 gigs), when I choose a Format of ‘Mac OS Extended (Journaled)’, I get a list of three Scheme items:


GUID Partition Map
Master Boot Record
Apple Partition Map




Which is appropriate for a flash drive?

I know the first one (GUID Partition Map) is for booting modern Macs. Booting is not an issue for my use of this flash drive.

The second one (Master Boot Record) is old, from the 80's DOS 2, for old PCs. Why is this offered as a scheme for "Mac OS Extended (Journaled)"? That really threw me, and prompted this Question.

The third one (Apple Partition Map) is for booting old Macs (68K and PowerPC rather than Intel chips).

Mysterious error when formatting higher item

No matter what Scheme I choose, I get an error when trying to format the item higher in the hierarchy in the sidebar to the left (Lexar USB Flash…), as shown in the screen shot above. Why?

If I select the secondary item in that list (Lexar), I can successfully do a format operation. But the "Scheme" popup menu does not appear in the panel. 

What is the difference between the two items in hierarchy? Does it make sense that I cannot choose a Scheme on the secondary one?
In virtually all cases, you want GUID Partition Map. Only choose a different scheme if you have specific needs that require that scheme. GUID Partition Map provides the best compatibility and is the most current scheme.

Without knowing what the error is, I can't say why you're unable to erase the disk. The top level item is the disk itself, whereas the lower level items are partitions. When erasing a specific partition, you're only changing that partition, not the whole disk. Since the scheme is for the entire disk and not one partition, it doesn't make sense to have the scheme show for a partition.