Although I have no first-hand knowledge of flash card electronics, let me try to detail my understanding of the subject.
First, aligning to 24 MB does align to all its sub-multiples.
But whether this is useful is another question.
Second, aligning the partitions to the erase block may make you feel better,
but what is important is actually the alignment of the files within the file-system.
As the file-system starts with its own internal tables, one has to force them to take up a multiple
of the size of the erase block, which is a far from a trivial exercise.
Aligning the file-system is therefore your next task after aligning the partition.
For example, read the struggle of Theodore Ts'o to achieve this for his SSD disk :
Aligning filesystems to an SSD’s erase block size.
Third, the term of "erase block" and its applicability to any particular flash drive
is in reverse proportion to the intelligence of the flash disk controller.
Modern flash controllers are getting smarter and even using compression, so the idea that
the operating system does know what is going on under the hood with that drive may be just a fiction
that may rather apply to the cheaper or older drives.
Fourth, block sizes are a function of the fabrication technology.
The AnandTech article
Micron Announces 16nm 128Gb MLC NAND, SSDs in 2014
has this interesting summary table, where block size goes from 512KB up to 8MB :
Fifth, some memory technologies use N-state "bits" called cells, so that one physical cell
may contain 1.5 or 2 logical bits or more.
The controller, again, does the job of translating this to terms
that the operating system can understand, which is of course pure fiction.
The article Flash memory card design has this to say :
In all SLC and MLC flash, the erase block size is a power-of-two size.
In TLC flash (Also known as MLC3, three-bit MLC, 8LC, or eight-level
cell), there are two possible layouts: An erase block can have either
three power-of-two sections, resulting in a total size of e.g. 1.5MB
or 3 MB, or it can have a non-power-of-two number of pages for each
level, 43+43+42 pages for a total of 128 pages per erase block.
Read this article to find out more about how flash disks report to the operating system
information that is basically false.
Conclusion: In the current jungle of evolving technologies there is no way to reliably
compute the real size of the erase block.
Tools like flashbench may be the only reliable way around for calculating that value,
but the results may be false for more evolved flash disks.
Imagine for example flashbench writing a block of 64MB zeroes that the flash controller
compresses into a few KBs, therefore flashbench actually really measuring the
transfer rate to the device.
In my own opinion, as partition alignment is counted as a "good" thing to do,
4 MB is enough (8 MB may be better for newer disks by the end of 2014),
and I wouldn't bother myself with more.
If in doubt, use flashbench, but keep an open mind as to the validity of the results.
Weird sizes reported by flashbench are probably the result of translations and optimizations
done by the flash controller on the flash disk.
The first partition (1 MB) is supposed to be used as BIOS boot partition.
But, as you can see, it isn't. All of your partitions are MS Basic Data partitions, type EBD0A0A2-B9E5-4433-87C0-68B6B72699C7. GRUB's so-called "BIOS Boot Partition" (a rather misleading name as this partition is not understood or used by the actual machine firmware) is type 21686148-6449-6E6F-744E-656564454649 (an invalid GUID).
That's the partition type that GRUB is looking for. You don't have one of that type. Hence the error message saying so. It's as simple as that.
Further reading
Best Answer
Yes, you're missing two things:
gdiskhas a non-trivial ruleset, too.) The EFI specification doesn't mandate that partitions be aligned to the first or last usable blocks. Indeed, it doesn't even, strictly speaking, mandate that the first usable block immediately follow the primary copy of the partition table.