[EDIT] - Now try using 'force' (--force)
If grub-install now proceed with message "Installation finished. No error reported." even with warning " Embedding is not possible. GRUB can only be installed in this setup by using blocklists. "
You should be able to boot up normally without that usb now.
If we do not need your install media, please clear up this install usb media.
Use gparted and do the 'create partition table' so you can use it like a normal usb.
You have a usb that was previously a iso9660 type usb.
If you cannot fix it through gparted, then do not use this usb.
I have an unusual usb too (not manjaro), but it says 'mac'; not gpt or msdos. Here's mine.
Warning: The driver descriptor says the physical block size is 2048 bytes, but
Linux says it is 512 bytes.
Model: Imation Atom USB Device (scsi)
Disk /dev/sdd: 16.0GB
Sector size (logical/physical): 2048B/512B
Partition Table: mac
Number Start End Size File system Name Flags
1 2048B 6143B 4096B Apple
2 334kB 2726kB 2392kB EFI
It says 16 GB. But I know it is only 4 GB. When I do os-prober or update-grub with this usb attached, it gives same error. but proceeded with all other OS's in other drives.
grub-probe: error: cannot find a GRUB drive for /dev/sdd1. Check your device.map.
Ultimately, if all else fail, I suggest you use another usb to make a new install media and and boot to manjaro and proceed to repeat the process.
But let us know, ya.