How to restore mbr on asus 701 via flash card?

Question

linux

There is an asus eeePC 701. When loading, it gives out a "missing operation system", and regardless of where it is proposed to boot from, from a flash drive or from a hard drive. Changing the boot priority in the BIOS did not help. An attempt to boot Partition Magic from a flash drive, the image of which was loaded by UNetBootin, also did not bring success. That is, the main problem is that you can’t boot from a flash drive at all.
I suspect that I mistakenly demolished mbr and I'm trying to restore it, but it's not clear how this is connected with the refusal to boot from a flash drive. I tried to put on it a variety of distros using the above utility - the same error.
The last time the system booted from a flash drive, during the act of manually re-partitioning, fdisk -l gave out two partitions sdb and hdc, while they differed in size by only a few hundred bytes, although there is only one hard drive in the netbook. The decision was made to change the sdb markup and reformat to Linux, after which an attempt was made to install Damn Smal Linux in sdb, which in turn was unsuccessful and the result was what I'm trying to figure out now.
What could be the problem? What ideas do you have, what materials can you turn to for a solution?
Thank you very much in advance for your replies.

Answer 1

The problem turned out to be completely banal, but nevertheless enough to take a couple of hours.
fdisk -l /dev/<flash_card_name>
showed that the card is not marked as bootable (boot)
This flag is changed by the "a" option in this utility.
Problem solved.